So i've always had this problem as long as i can remember but when i'm at higher speeds like 75-85mph and steadily cruising (not giving it gas just maintaining speed) there is a vibration coming from the rear of the car and i can even feel it in the shifter handle (it's a 6spd car). when i slow down sometimes it continues until it gets slower like 65mph and if i'm feeling the vibration and get back on the gas it goes away. What could this be. I noticed the rear pinion seal is leaking and i'm going to replace it and i got diff. control arms w/reloacation brackets that are going to be installed as well but it did it prior to lowering the car. I feel it could possibly be the drive shaft itself. Does anyone have any similiar problem(s) and/fixes for this!? It's soo annoying! and i just balanced my tires so i crossed that off the list of possible issues.

 

http://ws6.com/shaft.htm

most of the time a LS1 ds will fix the issue.

 

thanks so much. so after reading that i need to just buy a certain part (the part number listed) or just buy a LS1 drive shaft and it'll bolt right up fixing the problem?

 

pretty much, you most likely have the stock steel 2.5" (unbalanced) driveshaft. commonly known as "vibrashaft" a lot of the members here experienced it. Apparently there are no long term affects, besides the annoying vibration. however, when i got my car, its the first thing i noticed, and the first thing i replaced. There will always be some sort of vibration at high speeds, due tot he rear axle configuration in these cars. but your issue can be fixed.
4 nuts at the rear yoke will loosen the ds, you need to push it forward (towards the transmission) and then you can move it around, and pull it out. a very easy 15 min job.