Well I "solved" the issue.

I uncommented the dual extrusion code in my config.g, and the extruder started working.

Bafflingly, I tried commenting out the dual extrusion code again to see if the issue came back, and it didn't. So I'm running the same config.g as above, with no other hardware or software modifications and the extruder works fine. I'd tried multiple config modifications before to no avail, and rebooted it many times prior to this. So I have no idea what the issue was, or why it disappeared, but it's gone now.

@weed2all @Danal :
For all attempts to use the extruder, I have either used M302 or heated the hot end up to above 160 C. Additionally, it may be helpful to know that I am using the built-in stepper drivers. Moreover, all of the stepper motors for my machine are identical and configured the exact same way, except for the steps per mm.

@Danal My current config.g:

M550 PRMAXV32                              ; Printer name
M555 P2                                    ; Gcode Output Type
M552 S1                                    ; Enable Wifi
M575 P1 B57600 S1                          ; PanelDue Comm Setup
G21                                        ; Work in millimetres
G90                                        ; Send absolute coordinates

M569 P0 S0                                 ; Drive 0 goes forwards (X)
M569 P1 S0                                 ; Drive 1 goes forwards (Y)
M569 P2 S0                                 ; Drive 2 goes forwards (Z)
M569 P3 S0                                 ; Drive 3 goes forwards (E0)
M569 P4 S0                                 ; Drive 4 goes forwards (E1)

M584 X0 Y1 Z2 E3:4						   ; Map drivers to motors.

; set endstop configuration (all endstops at high end, active high)
M574 X1 S1 P"xstop"   ; X min active high endstop switch
M574 Y1 S1 P"ystop"   ; Y min active high endstop switch
M574 Z1 S1 P"zstop"   ; Z min active high endstop switch

M665 R144 L291.06 B135 H400 X0 Y0 Z0       ; delta radius, diagonal rod length, printable radius and homed height
                                           ; Y X Z are tower angle offsets
M666 X0 Y0 Z0                              ; endstop offsets in mm

M350 X16 Y16 Z16 E16:16 I1                 ; Set 16x microstepping w/ Interpolation
M92 X200 Y200 Z200                         ; Set axis steps/mm
M92 E178.4:178.4                           ; Set extruder steps/mm

M906 X1200 Y1200 Z1200 E1200:1200 I50      ; Set motor currents (mA) and idle current %
M201 X4200 Y4200 Z4200 E5000:5000               ; Accelerations (mm/s^2)
M203 X18000 Y18000 Z18000 E18000:18000           ; Maximum speeds (mm/min)
M566 X2000 Y2000 Z2000 E2000:2000               ; Maximum instant speed changes mm/minute

M950 F0 C"fan0"
M950 F1 C"fan1"
M950 F2 C"fan2"

M106 P0 S0 H-1                             ; Part Cooling Fan
M106 P1 S0.5 H-1                           ; Case fan
M106 P2 S1 T50 H1                          ; Heat sink fan

M307 H0 B0                                 ; Heated Bed (H2)
M308 S0 Y"thermistor" P"bed_temp" A"Bed temperature" T100000 B4388 R4700 H30 L0         ; Bed thermistor
M950 H0 C"bed_heat" T0

M308 S1 Y"thermistor" P"e0_temp" A"Hot end temperature" T100000 B4388 R4700 H30 L0         ; Hot end Thermistor
M950 H1 C"e0_Heat" T1
M563 P0 D0 H1                              ; Hot end (T0), drive (E0), heater (H1)
G10 P0 S0 R0                               ; Hot end operating and standby temperatures

;Dual Extrusion Code 
;M563 P1 D1 H1                              ; Hot end (T1), drive (E1), heater (H1)
;G10 P1 S0 R0                               ; Hot end (1) operating and standby temperatures

M558 P5 C"^zprobe.in" I0 A5 S0.05 R0.4 H20 F2500 ; Strain gauge probe settings
G31 P100 X0 Y0 Z-0.22                       ; Probe trigger and offset values
M557 R140 S30                              ; defualt bed mapping
M501                                       ; Load saved config values
T0                                         ; Select Tool 0
M375									   ; Load height map

I've got a Duet Wifi running RRF3 beta 3.

If a drive is designated an extruder drive, either via M584 or by default, it ceases to function. The a stepper connected to the drive will buzz, and shaft will vibrate a bit, but it won't spin, even unloaded. However, if I reassign the same drive as an axis drive, say X, everything works like it should. This is with no hardware changes whatsoever. Conversely, reassigning a working axis drive as an extruder drive causes it to exhibit the symptoms outlined above.

I've looked through the RRF3 docs and my config file, but I haven't been able to find anything that would cause this.

Any help would be appreciated.

I need some help figuring out why I can't switch extruder drives.

I'm running a Duet3D Wifi with version 3 of the firmware.

For whatever reason, the extruder on my printer recently stopped working. The extruder's stepper is still functional though, so I figured I'd switch from the E0 to E1 extruder driver. As I understand, the way to do this is to change the M563 parameter D0 to D1 in the config file. However, if I do this, all the UI elements for the extruder disappear from the Duet web interface. If I attempt to activate the extruder via g-code, I get a "No tool defined " error. As far as I can tell, the second extruder is fully defined in the config file, shown below:

M550 PRMAXV32                              ; Printer name
M555 P2                                    ; Gcode Output Type
M552 S1                                    ; Enable Wifi
M575 P1 B57600 S1                          ; PanelDue Comm Setup
G21                                        ; Work in millimetres
G90                                        ; Send absolute coordinates

M569 P0 S0                                 ; Drive 0 goes forwards (X)
M569 P1 S0                                 ; Drive 1 goes forwards (Y)
M569 P2 S0                                 ; Drive 2 goes forwards (Z)
M569 P3 S0                                 ; Drive 3 goes forwards (E0)
M569 P4 S0                                 ; Drive 4 goes forwards (E1)

M574 X2 Y2 Z2 S1                           ; set endstop configuration (all endstops at high end, active high)

M665 R144 L291.06 B135 H400 X0 Y0 Z0       ; delta radius, diagonal rod length, printable radius and homed height
                                           ; Y X Z are tower angle offsets
M666 X0 Y0 Z0                              ; endstop offsets in mm

M350 X16 Y16 Z16 E16:16 I1                 ; Set 16x microstepping w/ Interpolation
M92 X200 Y200 Z200                         ; Set axis steps/mm
M92 E178.4:178.4                           ; Set extruder steps/mm

M906 X1200 Y1200 Z1200 E1200:1200 I50      ; Set motor currents (mA) and idle current %
M201 X4200 Y4200 Z4200 E5000:5000               ; Accelerations (mm/s^2)
M203 X18000 Y18000 Z18000 E18000:18000           ; Maximum speeds (mm/min)
M566 X2000 Y2000 Z2000 E2000:2000               ; Maximum instant speed changes mm/minute

M106 P0 S0 H-1                             ; Part Cooling Fan
M106 P1 S0.5 H-1                           ; Case fan
M106 P2 S1 T50 H1                          ; Heat sink fan

M307 H0 B0                                 ; Heated Bed (H2)
M308 S0 Y"thermistor" P"bed_temp" A"Bed temperature" T100000 B4388 R4700 H30 L0         ; Bed thermistor
M950 H0 C"bed_heat" T0

M308 S1 Y"thermistor" P"e0_temp" A"Hot end temperature" T100000 B4388 R4700 H30 L0         ; Hot end Thermistor
M950 H1 C"e0_Heat" T1
M563 P0 D1 H1                              ; Hot end (T0), drive (E0), heater (H1)
G10 P0 S0 R0                               ; Hot end operating and standby temperatures

;Dual Extrusion Code 
;M563 P1 D1 H1                              ; Hot end (T1), drive (E1), heater (H1)
;G10 P1 S0 R0                               ; Hot end (1) operating and standby temperatures

M558 P5 C"^zprobe.in" I0 A5 S0.05 R0.4 H20 F2500 ; Strain gauge probe settings
G31 P100 X0 Y0 Z-0.22                       ; Probe trigger and offset values
M557 R140 S30                              ; defualt bed mapping
M501                                       ; Load saved config values
T0                                         ; Select Tool 0
M375				; Load height map

I've also tried D0:1 which yields the same problem.
Additionally, I've tried uncommenting the the dual extruder section of the config and then manually switching to the second extruder via T1. The first extruder shows up fine, but I can't switch to the second. Checking the tool set-up with M563 P1 gives a "No tool defined" error.

Any help will be greatly appreciated, thanks.