I'll start of by saying that I have been searching the forum for months and trying things that others have done to get rid of my vibration. I have a 1989 IROC with a 305 / 5 speed and Borg Warner axle. I have a vibration/resonance that comes on about 65 mph. It can be felt slightly on acceleration/steady speed, but becomes very bad when the clutch is put in.
I started at the driveshaft...bought an aluminum driveshaft from a newer camaro..same vibration. Got the aluminum driveshaft balanced just to be sure, and the vibration remained. So, I checked the pinion to make sure it was tight, and I can say that it is not loose. The tailshaft bushing in the transmission seemed a little loose, so I had a new bushing installed there....vibration remained. The car needed new tires, so new tires were installed with a "performance wheel balance", whatever that means. I'm pretty sure that the balance was not done with the wheels on the car.
Had the rear axle wheel seals leaking a little, so I replaced the wheel bearings. I got to looking at driveline angles, and noticed that the transmission was a little low. So, today I replaced the transmission mount and the torque arm bushing. The vibration/resonance is now the same, if not worse.

All in all, nothing that I have done has really improved this vibration. Short of having a shop tear into the pinion/ring, I'm out of guesses.

Anyone have any suggestions?

 

bent axle, bad rotor/drum
i had a vibration once started around 75 and got worse. like you i tried everything to fix it but nothing helped. i decided to buy a new set of rear tires, not that i really needed them but they looked old so i got a new pair. when i took the wheels and tires to be mounted there was about a quart of mud/dirt inside the old tire. after it was removed no more vibration