Originally Posted by VAEM
The vibration started after you had the gears installed?
Yes. Vibration free with the 3.27's and stock steel driveshaft. Re-balanced the steel (new u-joints) and got a bit better. Installed aluminum and got a bit better yet again. I used new flange bolts each time. I'm wondering about pinion flange runout as the shop I used for the gear install has a good rep. so I'm giving them the benefit of the doubt. It's enough to vibrate the rear view but certainly not as bad as with the stock driveshaft.
As an aside, there are a lot of posts out there on various boards about people running the Ford Racing 'J" aluminum driveshaft on 2001 IRS Cobras with T3650 as a direct fit. I decided to take a chance knowing there was at least 0.7" difference in length. Some may have had success but my personal experience is the Al shaft is 1" (44.5" stock vs. 45.5" centre line of u-joints) longer so food for thought anyone is thinking about that swap.