I have a 73-318 Cuda. I had a 3.91 posi unit installed and now that it does turn more rpms, I have noticed that there is a vibration in the car. I have check the driveshaft for balance, angle, new U joints, etc....When going down a hill and thrown into neutral the car is smooth as silk, which eliminates all the above. When under power it vibrates. Its not excessive but enough to make you not want to drive it at 55 mph. It has a newer rear tranny mount installed. Back when the tranny was rebuilt I had a re-manufactured torque converter installed. (No weights) That was before changing gears. Could that be the culprit? Only 2500 miles on converter. Any other ideas what this may or could be? Almost feels like motor mounts touching frame, but is not. Thanks in advance for the help. Jeff.