so i changed my trans cuz i blew the sycros for 3rd and 4th in my m6 and now when i drive the car it has a really bad viberation when i get up over 80 mph and im courios as to if any body knows what might cause it

 

Was the driveshaft torqued back properly? Only other thing I can think of is tires need to be balanced. Can you tell where the vibration is coming from?

 

did you mark the driveshaft before you pulled it? and put in back in the same spot?

 

ti would not have matterd if i marked the drive shaft cuz i put a differant trans in the car so no and it feels like it is the drive shaft but is hard to tell

 

You could try hand balancing it using hose clamps and moving the clamp around the shaft and seeing if the vibration goes away, or have the driveshaft professionally balanced (which is not very expensive - maybe $50, tops?)

 

sounds like the drive shaft ?

 

take it out and rotate it

 

I had the same issue on mine. I pulled my stock driveshaft and changed the U-joints. Unfortunately, while changing my U-joints, my driveshaft got squished too much in my vice, and I ruined it. Bought a 4" Aluminum driveshaft, slapped it in, vibration went away.

Check the U-joints first, make sure they're good. If they're fine, look into getting it balanced professionally.

If it's not the driveshaft, it's possible you could be having rear end issues. Check into the driveshaft first, as rear end difficulties can get expensive, especially if you can't work on it yourself.