My 99 Silverado Z71 will change into 4H or 4L. However, it will only move about 10 feet before the whole drive train binds up and it won't move. Switching back to 2H works but it feels like the whole system has been twisted like a rubber band and then released. Shifting into 4H on the fly causes it to sound like everything is coming apart and it locks down at that point. By way of history, I've had it since it was new in 1999. It has over 400K on it and until recently has been pretty much trouble free. In the last three years, I've replaced the rear differential, the 5.3L engine, rebuilt the transmission and today we changed the transfer case thinking that was the problem. But it didn't change a thing. I've reached the limits of my technical knowledge and can't think of anything else that would cause this. We took the rear end out of a wrecked Silverado. It works just fine. However,, I'm wondering if the transfer case is setup dependent on which rear you have. This was supposed to be a 373 locker because that's what mine was. But I can't be sure. Anyone run into this before?