Have your driveshaft checked.

Several years ago I mini tubbed my Dart, installed a new S-60 and a built to spec Dynotech driveshaft. I chased a vibration for five or six years and nothing I tried helped. I had the transmission out of the car a couple years ago and on a whim decided to take my driveshaft to a local shop to have it checked out. It was way out of balance and required a large weight on each end. My vibration is gone.

I'll never use or recommend Dynotech again, that shaft was over $450 and there is no excuse for it being out that bad.