Third Generation F-Body Message Boards

I've been fighting a driveline vibration for quite some time now, and am not sure which step I should take to help resolve it. To start off, I have done a T-5 swap from a 700R4, and have lowered my car with an Eibach Pro kit, so it's approx. 1 1/2" lower in the rear. There are no vibrations at all until I hit approximately 70 mph, and then I can feel and hear a severe vibration. I can feel the vibration coming through the shifter if I hold on to it. I initially thought the problem could be tires, but I just replaced both rear tires, had them balanced, and the problem still exists. Here are a couple of thoughts that I had. Please give feedback.

1. The driveshaft is out of balance. I am thinking of replacing with an aluminum driveshaft.
2. The transmission I put in seems to have a growl to it at lower speeds, which I think is the bearing on the output shaft. Could this cause the vibration at high speeds. Keep in mind, it is a severe vibration. Or, would anything from the swap have done this?
3. Would lowering the car have done this? I have not put an adjustable panhard bar on yet. Not sure if that would help this problem or not. Would the pinion angle be changed enough to cause a vibration like this?
4. I am running an aftermarket poly transmission mount. I think I read somewhere once before that this can cause vibrations. Is there any truth to that statement?

I'm not sure which direction to go first, and I realy hate to unecessarily replace a bunch of parts. Any direction you can give would be greatly appreciated. Thanks guys!