Slight vibration starting around 80mph after T56 install??

I just got finished installing an 03 Cobra T56 a few weeks ago, and now when I hit about 80mph I start to get a slight vibration. It's really not too bad around 80-85 and most people wouldn't really notice it, it's more of one of those "vibrating massage cushion" kind of vibrations and doesn't make any noise. But it get's worse the faster you go, and I had it on a back country road the other day and got it up to about 115-120mph and it got pretty bad and started making a loud vibrating noise. I did have to have my driveshaft shortened with the tranny install, not sure if that would have anything to do with it, or maybe the angle of the drivshaft now.

If anybody has any ideas/input it would be greatly appreciated.

Nope it wasn't there before I did the T56 swap. But I have Richmond 3:73's in the rear end and the rest is stock. They were supposed to balance the the drive shaft, or at least I paid them the extra $40 to do it. If it was the driveshaft out of balance, wouldn't there be a lot more vibration? Thanks for the input guys