I had this problem before and it turned out to be the dreaded trailing arm bearings problem,
go to the back of the car take a good few steps back and look at the camber of your back wheels, will porbably be /----\ rather than |----|
or jack the back of the car up and put it on axle stands, Hold the back wheel @ 3 & 9 O'clock and give it a wiggle see if there is some play.
|