Start by looking for a broken axle shaft. Jack up the rear wheels one at a time and try to turn the lifted wheel with the engine stopped and the transmission in gear. It should not turn. The axles were not nearly as durable back then as with today's steel alloys. Another possibility will be the universal joint under the torque tube ball at the back of the transmission. If it's broken, either of the rear wheels will turn, but there might be some grinding or rattling inside the pivot ball. Both of these items can be checked with a minimum of disassembly. Of course, the cover plate at the back of the rear end can be removed for a quick look-see at the gears. Start at the back wheels and work your way forward until you find something broken.
Jerry
