I drive an 88 Ford Mustang. It is well taken care of, with no racing or hot rodding. It has 160,000 miles. I recently replaced the front and rear bearings. I have also installed new tires that were carefully balanced. At approximately 30 mph and 75 mph, I get excessive vibration. It feels like it is coming from the rear of the car. I wonder if it is a bad u-joint? I have also been told it might be the motor mounts. Does anyone have an idea to the source and correction to this vibration?