the u joints didnt turn in 2 wheel drive. i am guessing that since the axel shaft goes into the hub and when you drive down the roadthe hub spins freely arount the axel shaft. with it having play i am guessing that is what caused it to vibrate. i cant think of any other reason. and it went away when i replaced the seal. my old seal was totally destroyed and there was about a half inch of movement in the shaft.