I had the same thing happen to me before on my 91. I rebuilt the rear end and swapped gears. Car drove very smooth before. My gear pattern was good and pinion bearing preload was adjusted within spec. The car had a vibration at 110 -130. The ujoints checked out fine but I went and replaced them anyways. I even tore the rear end apart again and did it over. Nothing worked. I was going crazy!! I put in an aluminum ford racing driveshaft, the car was smooth as a foxbody could be.