Do you need an alignment after changing gears? I went from 3.55 to 4.56 and when driving I noticed it vibrates around 40 mph then goes away before or after. I did have a lift kit and the technicians probably had to drop the cross member or something to get to the front diff. Im not sure if the shaking is suppose to be common on new gears.

 

No, gears have nothing to do with alignment. A friend of mine experienced the same thing.... vibration after a gear change on his Mustang. The culprit was a u-joint.

 

could be excess backlash variation, will have to determine if it is from the front or rear,
then backlash will have to be checked in several places on the ring gear. also check
ring gear runout, if it not installed evenly on the carrier it can cause this. can't remember the spec offhand but should be like .004in or less