When if locks up, is the ABS lights on?
Were you having issues before 4x4?
Usually rear end locking up is because the ABS sensor in the diff is failing. Sometimes cleaning them works, but if you still have the stock sensor, you should replace it. If the ABS light is on, a shop can read the ABS code, and that will definitely help narrow it down. If you get the rear speed sensor code even with a new sensor, it's mot likely a broken wire between the frame and axle. This happened on both mine and my brother's.
2000 E450 dually V10 wagon