The inner bearing that the axle stub goes through can go bad and cause the axle to creep . Problem I had was my right u joint had one tied up bearing cap, so when the axle creeped to the right spot the steering wheel wouldn't turn . You'd pull hard snap it back and the truck would be all over the road like the death wobble we read about on hear now n then. Changing the u joints is not bad, but they are big so my ball joint tool didn't work great but it worked. You also need the seal installer for the seal behind the unit bearing " hub " or something to do the job.