HELP! Car rumbles/vibrates on acceleration in 5th gear from 70-80 mph

I know I`ve posted this before, but I have some more spcific information.
The car in question is a 94 Eagle Talon TSI (AWD Turbo) with 93,000 on the ODO.
As I accelerate in 5th gear, as soon as the car hits 70 MPH it begins to rumble/vibrate not unklike what happens if you try to accelerate in a high gear while going to slow. You hear it and you feel it in the seat and the floor, but not really on the shifter or the steering wheel. That leads me to believe that it isn`t the front wheels or the tranny, but I could be wrong. Feels like it`s coming from directly below the driver/passenger. The rumbling seems worst at 75, but at 80 MPH the rumbling dissapates and is gone by 85. This only happens in 5th gear and only at that exact speed window. In 4th I can accelerate from 40 MPH right past to 85 with no rumbling. I would just ignore it, but 75 in 5th gear is highway cruising speed. Something is out of balace or loose or something and that speed/RMP combo is just the right frequency to cause it to vibrate.

shows itself in fifth gear, problem`s probably inside the gearbox. Any good specialists near you you can go and chat with?