HELP - vibration in the driveline

When i am out driving, if i get above about 40 miles an hour, i get this heavy drone/vibration in the car. With the windows up it feels like your head is going to implode on the freeway. So i stopped driving it on the freeway. Turns out i have to do some freeway driving tomorrow so i want to fix it.

So the symptoms are

- vibration/drone at above 40 mph, gets worse the higher speed you go
- only vibrates with foot on the gas, at 60 i can take my foot completely off the gas and the vibration completely goes away
- i would guess the vibration resides around my area of the car, motor/tranny area, not back in the rear end, it just seems to be right under me

If anyone has experiance with something like this, or at least some ideas, please let me know, i am in DIRE need of solving this problem... my fishing trip depends on it.