You're certain its from the back? If it was the motor/trans mount it would be coming from the front obviously

well i thought back because I feel nothing in the steering wheel..its in the seats..so whatever is vibrating is causing a resonance to travel through the floor and into the seats..the whole car shakes basically but I just didn't feel anything in the steering wheel so I assumed it was from the back, but I could be way off on that assumption.

See video here: https://www.gmforum.com/showthread.p...91#post1537191

ive had the same problem mine was a rear rim that was out of balance if i where u see if u got a bent rim or check the rear tie rods or rear balance the rear tires

When I got home today I decided to take the car out to go put gas in for the upcoming trip. The vibration continued to drive me crazy and still in my mind it seemed like a wheel balance issue. After going and having them balanced twice already, I just didn't get it...

Anyways I went with my gut instinct and took it to another place to have them checked to be absolutely certain...sure enough all were out of balance AND one even has a slight bend in it! I specifically gold the last shop to check for one but they said they were all perfect.. Well they road force balanced them and the vibration is gone!

Moral of story, don't trust what a shop tells you or that their equipment is properly calibrated. Um going to be having a word with the manager of the first place.