[merc123]

3.73's installed by Ford. I put close to 30,000 miles on them before something gave way in the tranny (bearing?) and had 0 whine and 0 vibration.

[fast97gt]

I will, alrite people I swapped u joints in my 97, it had no vibes at all, even 30*** miles after the gear swap. However when I did new u joints I got a vibration, I had put the ds on the pinion flange in the wrong location, turn it to the next set of holes and fixed it. Now I'm agreeing with the ds not spinning faster theory, it spins quicker through the rpm range but your ds is spinning the same speed once you hit top gear, now hold on ok, here's my theory, my 5th gear is like my 4th gear, so with me in 5th with my 4.10s is like doing 70mph in 4th with stock gears. So yes you can say its spinning faster but its not the same. So if had no vibes at 70 in 4th with stock gears you should have none now. When they had the ds off they could of not put it back in the right holes
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[Kilgore Trout]

Originally Posted by usaf_branham

i get the drive shaft imbalance possibility, i dont think its that, just for the simple reason that

before 75mph = 2000rpm - no vibration
now 2000rpm = roughly 55mph with vibration

sooooo if the driveshaft was imbalenced before, it would have vibrated then at 75 or higher. I am wondering if the shop might have accidentally installed one of the bearing races in a little cockeyed, or if the lash is set too close. i watched him check the backlash and it was .10 which is good, but if the gears are heating up and expanding, maybe that lash is closing up and causing the roar. What do you guys think?

If they did not mark the drive shaft prior to removal then its a crapshoot as to whether they will bolt it back on the SAME WAY IT WAS. These kind of sloppy mistakes indicate a poor shop.

ALWAYS MARK THE DRIVE SHAFT PRIOR TO REMOVAL so it goes back on the same way it was.