the vibration could be from the size difference in the two axles. a buddy of mine had this EXACT same problem on his 96 coupe. he replaced the driver's side (twice) with a brand new axle from autozone, but the car began vibrating. we compared it to the stock axle, and the autozone one is much thicker. when he put the old one back in, the vibration stopped. he ended up just putting a new one on both sides (both from autozone) and it solved the problem. compare the new axle to the old one (if u still have it) and see if there's a noticeable size difference in the shaft.