Vibration at higher speeds

[Nickd1212]

I recently installed a set of 4.10 gears. Took my time (three weekends) and had everything adjusted perfectly. Got it back together and went to break in the gears. No issues, everything was great. Held it at 55 for most of the break in. No vibrations, grinding, whining or anything. Took it out the other day and went on the highway. Still good up until 55. Once I hit 60, I started getting a bad grinding noise. Not sure what is causing it. It sounds like it's coming from the whole car, not just the rear.

My question is could it be possible the gears are out of spec or would that show itself at all speeds? What else could cause grinding at certain speeds?

Little bit more info... When I first did the M6 swap, I could get into all gears easily. It progressively got worse until I couldn't get it to go into any gear. Replaced the leaking slave and it helped a little, but didn't get me back to where I was in terms of effort needed to go into gear and it has been slowly getting worse again. Could this have any effect on the grinding/vibration I'm having?

Any ideas on where to start looking?

[Injuneer]

Stock steel driveshaft? Vibration at high speed, with 3.73 or 4.10 gears is a common problem. Yours sounds a bit extreme though, with the grinding noise. Did you check the condition of the u-joints?

Am I correct in assuming it's an F-Body?

[firebird_1995]

Another thing to check would be the pinion angles if you've adjusted them as well as the driveline angle (should be measured on the transmission output shaft seal but can be measured on any flat spot on the damper). Do this while the car is on the ground. You may have a softening transmission mount that is causing a change in your driveline angle

[Fastbird]

I'm with Injuneer on the vibration if it's a stock driveshaft.

[Nickd1212]

Yes, she is a 95 Z28.

It shouldn't be the trans mount. I put in a new one of those when I did the swap. U-joints seemed fine, but it wouldn't hurt to replace them. What would I do about an out of balance driveshaft? Can it be balanced or does it need replacement? Pinion angle should be good, but I don't really have a good way to check it.

[96LT1355Z28]

Drive shafts can be straightened and balanced, and it'd be a good idea to replace the U joints while it's out. You mentioned trans problems earlier, did this problem (grinding and vibrations) not exist at 60mph and up till after the gear swap?

[Injuneer]

The problem with the stock steel driveshaft is usually not balance. It is the "critical frequency" of the shaft, the RPM at which it experiences destructive whipping from harmonic vibration. The problem was so common that GM issued a "technical service bulletin" (TSB) that allowed people experiencing the problem to get a free replacement with a 1LE aluminum shaft, at least while the car was still in factory warranty.