Symptoms:

Vibration is present with clutch in or out
Vibration is present in or out of gear
Vibration is present at 65, goes away at 70, and is really bad at 75 and higher.

I can feel it in the floorboard, through the shifter, and in the clutch pedal.

Is this the driveshaft, trans, or clutch? With 4.10 gears I figure the driveshaft is a good guess. However I can feel it in the clutch so I don't know if that is it or not. Any help is appreciated.

 

I had it at 80 when I had 4.10 gears. Alum driveshaft helped ALOT, but there is still a slight twinge at 100 just cuz I know now what to feel. Some of our cars just have the problem, GM took a lot of heat over it. But over 100 mph there is so much going on I don't notice anymore.

 

Oh, I left out. Large part was the $%^#$%#% install on the gears like not replacing the side bearings and not pressing the pinion seal right. New install corrected half the issue.

 

I'd say the gear could be it but I really hate to think what that means. I'm on my 2nd gear set now and hate to throw more cash into this POS stock rearend.

 

I used to have a very similar issue with my car. I went thru everything from lca's to a cf driveshaft to a brand new tranny from gm (and a whole bunch in between). After all of that, i had lca relocators installed, the car drives smooth now. Make sure your geometry is correct, mainly pinion angle. If you have upgraded suspension, try lca rel. brackets. Good luck...

 

This problem is an unbalanced driveshaft. Most people with the problem feel it at its worst between speeds of 90 and 120 MPH. Others claim they can also feel it by slowly bringing the RPMs up to the 1800-2000 range with the car in park or neutral. But that isn't necessarily the same problem. Some even start to feel it as low as 60 MPH. It usually starts with a mild resonating vibration you can feel through the gas pedal. Then in at 80 - 90 MPH, the vibration intensified and feels as though it's coming from the center of the car.