[MEDIK418]

I've found this on three t-5's so far and all had the same sort of vibration you describe. Try this, jack it up and check the lateral play on the slip yoke where it goes into the transmission. The tailshaft bushing is prone to extreme wear, especially if it ever ran low on fluid. When it does, it eats the slip yoke up and causes a lot of play between the yoke shaft and the bushing. Just a few weeks before we took the tranny out of the green thing, I found the bushing completely out of the transmission riding on the yoke. Once the shaft begins to eat at the bushing and the gap developes it will vibrate like mad, getting worse as you go faster. Try to wiggle the yoke only. If there's any play, you need to look into replacing the bushing (easy to do) and maybe the slip yoke as well. The bushing is really easy to install and costs about 4 bucks. The slip yoke is about 60 bucks from Denny's Driveshaft (the cheapest I found it) and 100 bucks from Ford and most other places that handle them.
FYI. . .this bushing is vital to keeping the output shaft centered in the transmission. If the problem persists, it can damage or destroy the output shaft bearings and the fifth gears.