Sounds to me like you have a axle vibration do to one of two things. The housing is bowed or not square in car. I had a 4x4 truck did the same thing . The rear housing which was a 9 1/4 was bowed. I found the problem after replacing the complete rear after the center pin fell out and destroyed the housing . After replacing the rear it was like a new truck. I dumped many dollars into it before this trying to fix the vibration. Check the housing and the 4 wheel alignment. This will be magnified with a sure grip mine was. try a open diff and see if it changes.