Check for a broken axle shaft. Jack up one rear wheel at a time, put the transmission in any gear, and try to turn the raised wheel. It should not move. If either wheel can be turned, try pulling out on the wheel and tire. If an axle is broken, the wheel, brake drum, and axle shaft will move out several inches. Check both sides. It's a quick, easy test that can avoid a lot of unnecessary disassembly. Broken spider gears in the rear end can cause the same problem, but those will usually make some expensive noises as the driveshaft turns.
Jerry
