fast1980camaro

Going down the highway car vibrates like hell, not tires they are all brand new with balance, but if you push on the drivers side rear of the car the body moves about 2 inches off the rear end, the passenger side rear of the car is fine though. Thinking it probably a bushing somewhere but I wanna make sure I'm replacing the right ones, just need to get it right till I get all aftermarket suspension, This is on an 89 I-roc, this is the car I am driveing while finishing my 80 camaro, as soon as its done then the 89 gets a full refreshing. Performance is everything. Mustangs Suck.