Had a vibration problem coming through the T5 shifter at fast highway speeds- 80-85 and higher. The vibration increased significantly (scary) if shifted into neutral or the clutch was pushed in at speed. No other noise, just bad vibration through the shifter. Vibration still there if engine was shut down and coasting at 80+.

This occurred after a large repair project where the car was apart and off the road for 4+ months. The rear was dropped down to change a rusted gas tank, fuel pressure and return lines from tank to engine, fuel sending unit, 9 bolt pinion seal, new KYB shocks and struts, 4 sway bar end links, ball joints, tie rod ends, idler arm, centerlink, T5 output seal, steering wheel, Spohn subframe connectors, wonderbar, ľ driver side floor pan replaced (due leaking t-tops), new exhaust from the manifold back with Magnaflow dual cats and Hooker cat back, rug, t-top seals, headliner, front seats, and a lot of small stuff. In the process of doing some of this work, the driveshaft was supported, and at one time it fell off the support and slammed down - cantilevered off the transmission extension.

Because so much was changed, I did not know where to start. Did the transmission main shaft get bent when the drive shaft fell against the transmission? Did the driveshaft balance go out when it fell? Did the rear get messed up when the pinion seal was changed?

After talking to a few experts, the driveshaft seemed to be a likely culprit and a relatively easy thing to try. The old steel shaft was replaced with an aluminum 2001 LS1 shaft. Unfortunately, the vibration was worse with the aluminum shaft! The aluminum shaft was checked at a driveline shop and the tech said the balance was great- better than most shafts when brand new. The transmission mount was good and the output bushing on the trans seemed to be OK- only a little play.

It seemed the next likely things were something in the transmission or 9 bolt rear.

The pinion seal had been changed using the method of putting an index mark on the nut and the pinion and counting exposed threads, so the nut could be put back on in the exact same position and the pinion bearing preload would hopefully be the same- without having to take the whole rear apart to use a new crush spacer. I now began to suspect that something was not right with the rear. I could not feel and play in the pinion bearing, so I did not how bearing preload could cause the vibration. Lacking other good options, I went into the rear.

The rear was torn apart and even though all the bearings looked good, I installed new pinion bearings, with a new crush washer and set the bearing preload to spec. Pattern check looked good.

I took it for a drive and the vibration was gone!

I believe that the bearing preload had been on the low side due to the method of changing the pinion seal. Even though there was no play that could be felt, pinion bearing preload must be a critical setting on this 9-bolt.

When changing just the pinion seal without tearing the rear apart, GM recommends measuring the preload of the whole differential prior to loosening removing the pinion nut, then during installation, torqueing the pinion nut until the same preload is achieved. In retrospect, I should have used this method, rather than using the index mark on the pinion nut.

I know many people are successful at changing the pinion seal by making a mark on the pinion nut, but in this case it did not work.

Luckily, the ring and pinion looked good- no damage was done by the vibration.

Thirdgen.org has been a great reference for solving problems, I posted this in case it might held someone someday.