Ever since I put in my poly trans mount I've been having an issue with the rear end "jumping" at highway speeds. It seems like every little bump shakes the rear of the car violently. It happens at all different speeds (55-85) and rpm ranges. I checked the height of the poly mount and it is the same as the stock mount so the pinion angle should still be good. I thought it was just the worn out stock shocks, but I just put on a new set of Bilstein HD's (still stock springs) and it didn't help at all. If I switch back to the stock mount it goes away almost entirely, though I can still feel just the slightest bit of vibration. Any ideas what it could be?

 

are you measuring the mount height installed in the car or side by side with both of them out?

the old mount may have be compressed a bit due to age when its on the car, and the new one may be changing your pinion angle a bit.

 

I measure them out of the car. The stock one is a new GM mount with around 500 miles on it. I guess it could be compressing some with the weight of the trans on it.

 

might be worth it to check the pinion angle with each mount to see if thats your issue.