I was wondering, my car seems to run flawlessly from 0 to about 70 mph. Once i hit 80 or above its starts to vibrate like crazy, I had to backdown from a 80 something mustang because i thought my car was gonna fall apart. I didnt know if this was a driveshaft problem. I just had everything in the pumpkin replace and put in 4:10's. All i have done to the car right now is flowmaster exhaust, slp cold air intake, hypertech tuner, the 4:10's, and a hyperformance clutch.

 

Sounds like to me it could be your tranny mount or a drive shaft problem.

 

I'd say driveshaft...

I spent all last year fighting the same problem, I even tried and LS1 shaft and even that didn't work...

Got a new Denny's DS, and now she's smooth as glass to 140+.