General Maintenance & Repairs - Drivetrain Vibration

So heres the deal its an 99 trans am 228r cam 4.11 gears m6 centerforce 12" clutch chromoly driveshaft (which was bought new) I have been trying to chase down this vibration car has been to two different shops and nobody can pinpoint it. Driveshaft has been high speed balanced, new boxed rear control arms tires and wheels are in good shape.The car is lowered with eibach springs and the rear has stock springs without iso's. The symptoms are the shifter vibrates like a dam jackhammer when going 40mph in 5th gear the vibrations comes and goes and is only noticeable at certain speeds I can also feel it in the s actual car and not just on the shifter. Mechanics have checked for bent axles and all that but said they look fine. Last mechanic told me it was either my driveshaft or my transmission but getting my tranny rebuilt wont be a cheap what if and especially if it doesnt fix the problem so any ideas? .

Wheel bearings?

did one of your wheel weights fall off?

Motor and trans mounts?...

New motor and trans mounts. Wheel bearings are good. And wheels are fine also

What kind of mounts? Poly...? I just got rid of poly and my vibration went away. I rebuilt the trans, new clutch, new drive shaft, and rebuilt the rear end with new axles... none of that did it. Put an AC Delco transmission mount on it... fixed. Go figure.

Poly mounts. I tried putting rubber tranny mount and it amplified the vibration x10. The vibration comes in what feels like thru the shifter and the stick rocks back and forth horribly. Its also felt in the car itself not just the shifter.

Loose motor mount???

Checked them and they are good also. I noticed most of the vibration coming thru is noticeable in deceleration and at cruising speeds. I have seen the 5th and 6th gear vibration thread but I am trying to eliminate other possible options before tearing into the tranny.

Very odd. Decel and cruising speed stuff is usually driveshaft or a wheel.

is your driveshaft centered with the clips on the u joints??