When you stop and hold the rear wheels, does the gear continue to turn?
If so, the problem is in the rear, spider gears, pinion, axle.
But if the gear stops, the problem is forward, clutch, or transmission.
I'm thinking there is a coupling on the shaft, down in the belly, just behind the pump, that is designed to break before something else does. Been a while since I've been in there, but might drain the oil down so you can see what's happening.