severe vibrations

Need some advice on purchasing parts over the winter...

My stang starts to vibrate pretty bad once I reach 80-90 kph.

When I press in the clutch it stops. When I let of the gas, the vibration goes away slow as my speed decreases.

I can drive the car around fine, but as soon as I reach highway speeds it vibrates like crazy.

Gears 1, 2, and 3 do not vibrate at any rpm ranges. It starts to vibrate in 4th just before I would switch to 5th. But by this time I've reached around 80 kph. So, it doesn't seems like its engine or tranny related...any suggestions would be helpful.