I have a 91 camaro rs, 305, 5 speed.
I have a vibration around 40-45 mph it vibrates in neutral costing and in gear, really notice it in 4th gear off throttle down hill. Have checked wheels are not bent, checked u joints they are good Also around eighty it starts to make a noise

 

The noise around 80 is probably the drive shaft bowing from the high RPMs that it's turning. This is a common problem with rear wheel drive cars.


The lower speed vibration may be from a loose pinion nut or worn pinion bearings. I would have first guessed the u joints, but you already covered that. Also check the transmission mount. They cause similar problems often on these cars.

 

Simple stuff first.

Confirm that the wheels are balanced.

Look at the tire tread. Confirm the tires aren't cupped or worn in a weird pattern.

Try swapping the tires front to back. See if the vibration changes.

 

I just put a new transmission mount on and it's still fine I haven't checked the balance of the tires yet but the tires aren't cuped and the tires didn't no excessive wobble or bounce to them

 

I checked the pinion for play and the backlash seemed alright I balanced the tires and rotated them, checked for any play anywhere and all the bushings in the rear it helped but it still has a slight vibration and still has a noise around 80

 

If it has a 7.5 10 bolt the axles could be bent. I have that problem on my 84 camaro, i had bent axles so i changed them and within a week they were bent again. Did some more checking and my rear end housing is bent so I'd check into that.

 

Mine did that before. I found fix a flat in both the back tires causing them to be unbalanced.