I just replaced all of the bearings in the differential in my 2004 Mecrury Mountaineer. I did not replace the ring and pinion set. I now have a bad vibration above 50 mph. I just replaced the u-joints, and the rear wheel bearings are less than 4 months old. The u-joints are in phase and everything looks normal. The diff. doesn't get hot but it has some noise. The diff. store where I purchased the bearings said the diff. can't cause the vibration. Help!