I have an evil vibration coming from under the car in 4th and 5th gear at 60mph +. The faster I go the louder it is. It doesn't occur while accelerating but becomes apparant when decellerating. Once I'm on the clutch going this fast, wow, hard vibration, shakes the whole car, but stops after slowing below 60 or so. I've only noticed it in 4th and 5th. Its a 1991 with 3.73 gears. Only has 100,000 miles on it with only bolt on upgrades like headers, pulleys, intake, and rearend. I've checked everything I know. Anybody had this problem? or has any Idea how to fix? Some say drive line, someones else said throw-out bearing, but I dont think its tranny involved. Please help, Thanks