Tilt the bed and have somebody move the throttle pedal. Examine the movement where the throttle cable moves the lever on the fuel pump at the engine.

You need to determine if the issue is either:-
1. At the pedal.
2. In the cable.
3. At the pump.

Once determined somebody may be able to help you.

 

Have similar problem. 2013 Trans Mule 4x4 Diesel throttle won’t return to idle position when accelerator depressed. Not stuck, just won’t return. Looked at exploded views, can’t tell if there is a return spring on lever by injection pump.