I had the exact same vibration in my car. I finally narrowed it down to the tailshaft bushing in the T-5. A friend of a friend owns a tranny shop and let me borrow the special tool to remove the bushing while the tailshaft is installed in the car. It was very simple. Remove the driveshaft, insert tool, tighten bolt in center to draw out the old bushing, then pound in the new one until the tool bottomed out. This probably took 15 min or less and fixed my vibrations. If you can find the tool it is a very simple fix.