odd vibration at thruway speeds

[JAM]

i have a strange vibration at thruway speeds (50-70 mph)

it will feew like my diff is vibrating for 4 seconds or so, then be smooth as silk for about 4 seconds, then vibrate for 4 seconds, over and over again. guibo? diff? any suggestions? thanks

[jlevie]

The could be balance or roundness problem with the rear wheels. I'd want to rules out the wheels/tires as a cause before moving to the drive train.