What other power input components that drive the rear axles would be most likely to have failed which might prevent power to the differential? Does this seem to include such things as stripped splines on input shaft, broken ring gear, stripped pinion gear, etc? Is the ring gear bolted to the carrier or an axle flange?
Again from the parts diagram the drive shaft connects through the transmission to a bevel gear on a shaft with the brake. There are gears on this shaft that seem to drive the rear differential, which only drives the rear wheels. Whatever broke has to be downstream of the brake shaft because everything else works. So the ring gear or any of the differential parts could be bad.
I did raise the rear of the tractor and turned the wheels. When I did this I found they rotated freely and totally independently, regardless of the brakes or the diff lock. I was surprised that I heard no clunking, groaning or grinding noises when spinning the wheels.