Try to take the drive belt off the rear differential, if ti will start and run then your trouble is in the rear differential, if it still wont start and run witht the drive belt of then your trouble is in the bevel gear box. Were you changing gears allot when it quit driving? I have seen old tractor rear end's get stuck between gears but that usually only happens when you are changing gears in a hurry or get rammy with the shift lever.
Start with the small stuff, take the drive belt of, if it runs then jack up the rear end and try to spin the wheels, I the wheels spin freely try to spin the input shaft. This sounds like you may need to take the rear apart to fix it which is not to bad a job. You may have pulled the side of one of the shift gears of, I had a friend who did that by shifting to hard, there are little plates on the sides of all the shift gears so that when you put the trans into neutral or got to change gears your shifting fork hits that plate and pulls the gear into place.
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