well here goes all info is current in sig for explaining purposes.
im having a bad vibration at 75 and above it gets worse with mph increase. also if i push in the clutch at that speed to shift the vibration will go nuts and i feel it in the clutch pedal. I can go 74 and lower and it's fine. also in in all gears around 30 i get a bucking sensation, very hard to describe car seems to be lurching. i had my car at flp couple weeks ago and they bled the clutch and adjusted the exhaust. that helped a little. I also have an aftermarket driveshaft that is balanced to 3000 rpms.
also in neutral it will vibrate while sitting still at 3k and above not to bad but its there. i was told by mcleod that there street twin is balanced so i just put it in.
Any help will be great sinc i have a 150 mph + car with a 75 mph limit due to it vibrating my a$$ all over the place

 

I have the same high speed vibration problem and have never been able to pin point it. 65 mph + is hell

 

say you were to shift at that speed would the vibration increase or decrease ?
also i noticed yesterday that mine only vibrates when i let off the gas cause if i get on it again the vibration will go away

 

ya sounds like the same thing, if i push the clutch in put it in neutral it gets worse. seems like its not so bad when there is a load on the drivtrain (decelerating or accelerating)

 

Bandit: Did you ever figure the vibration out?

I have the same prob.

 

I HAVE THE SAME PROBLEM to if yall figure out what it is please post it.

 

you any of you checked your pinion angles??

 

Same thing here. M6 w/ stock driveshaft and 3.42's. With any engine load it isn't too bad....but when you let off at 70+ its pretty bad. When I run up to 100 or so and start to decelerate it's vibin' like crazy and noisy too.

 

no i haved not? but i have been told this might be the case. i have a strange 12 bolt and aftermarket 3.5 inch chromoly shaft balanced three times. as well as the tires. even took my buddys tires off his ta and same result so it's not the tires.

update on mine though i just found out both my axle flanges are bent pretty bad. wobbling real nice. i am getting new axles and bearings soon i will post let you know the results of this.
stupid 2500 dollar rearend