suspension issues?

I purchased my 2000 gt from a customer at the body shop i work for. It was wrecked but only cosmetic damage. i have noticed when i drive it that the rear end is very squirly when under moderate to heavy acceleration. almost like its not tracking right. I have measured all points on the undercarriage and the car is square. no unibody or suspension damage visible. i checked all bushings on the rear suspension and they were recently replaced at some time before i bought the car. has anyone else had this issue? i'm kinda stumped. i don't want to start throwin parts at this because that can get expensive. any tips? thanks