EL_CACK:

Could be a number of things. Pull the engine, check the spur - make sure there are no teeth missing, check the clutch. Next hold the front wheels and push the vehicle backwards on the ground - if the back wheels turn, blown diff in the rear or front. Hold the spur and try the rear or the front to see which is blown. If both are OK, but the dogbones turn free to both the front and rear - the center shaft on the spur is sheared. If you keep trying to tighten the slipper, but it just turns and turns - this confirms the sheared shaft.

If all this stuff is good - I don't know - you've got an enigma on your hands.