Fox Vibration After Gear Change

Your driveshaft could be out of balance now and going to a lower gear will emphasize this. Try mounting it 180 degrees and see if that fixes the problem. If so, mark the driveshaft and rear end flanges so you will know which side goes where. Normally, There is a yellow dot that tells you which side goes where.

Couple of things if that doesn't work

what was the tooth pattern on the coast side of the gears
was the pinion gear depth set correctly.
The crush sleeves are a PITA - can you tighten the nut more with an impact
Pinion angle should be about 2 degrees
Check the weight on the rear end- is it there and tight
output bushing on the trans that support the driveshaft yolk- do you have a lot of play in the front yoke

mikestang63 said:

was the pinion gear depth set correctly.

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This is my guess. How did you check the pinion depth?
x2 on tightening the pinion flange with the impact

Agreed with Mike. Same thing happened to me after my gear change, only the shop that did the change didn't tighten the bolts on the driveshaft all the way. They ended up backing out causing the vibration.

My bet would be the backlash is fine. I agree with Mikestang63, check the driveshaft first. I had the same isssue when I went from 2:73's to 3:55's, installed an alum. driveshaft and smooth as glass.