I have a Trans-Am 1994 LT1 Engine vibration at 2,800 rpm?

I have replaced torque converter, flywheel, harmonic balancer, checked all accessories, I have had it to 4 mechanics no one can seem to find it. It is so severe that it rattles the change around in the ashtray and makes the mirrors vibrate so much that you can't see out of them. I've put in new motors mounts, transmission mounts. I've had a couple mechanics tell me that my aftermarket ehxaust puts off a frequency at that rpm and causes the vibration. Anyone ever heard of that??? Its 3 inch Borla exhaust. I've put almost 20,000 miles on it with this vibration and nothings came apart internally in the engine yet. I bought the car with the vibration so its always been there since I've had it. Any suggestions or ideas would be greatly appreciated.