Hi, I'd agree with Harry. The essential thing is to determine where the problem is first. Start off by disconnecting the drive shaft at the rear end. Make sure that you have the 4WD engaged, Support the driveshaft and move the tractor. If the drive flange turns you will have drive to the shaft. If it does not the problem is in the middle region of the tractor. Try it again with the 4WD disconnected. If the flange does not turn all appears to be OK in the middle region of the tractor. Make sure that there is no movement from oil drag.
Turn the driveshaft by hand. If you can turn it the problem is at the front end. Jack each side up seperately and spin the wheel. Listen for any strange noises. I'm guessing that it would be a crownwheel and pinion problem. It is essential that you inspect it and make sure that nothing is broken/can break and possibly punch a hole in the casing if it gets caught up.
DavidP, South Wales