could be it's had a bang and the body re-pair has not been that hot, the stub axel has got bent, or the rear arm bushes are knackerd. I would never ever reccomend puttin spacers on the back of the car, with out doing the same to the front, but putting one on just one cornor is a hidious and dangrous idea, it will royaly phuq the handling when you really need it not to (ie excessive cornor speed etc.)
So, check;
bodywork
bushes
axel alignment
see if anything else is bent.
Cheers
Mark