1994 Cadillac Fleetwood Rear end

I was recently advised that the rearend needed replaced due to damage. After the rear end was replaced and as I am traveling at around 65 to 75 mph the car sometimes shakes violently with noise. And sometimes as I come off the accellerator all the way then depress again the vehicle seems to rock. It feels like only one side of the drive wheels are working on the left side. I took it back but I think I am too much of a nice guy because the line of hog----i was given did not make sense.