You have answered your own question really Kramer,
It could be a knackered shock absorber, or possibly a broken spring, try bouncing the back of the car on each side, If the shocker has failed, the car should bounce up and down quite freely on that side compared to the good side which should feel firmer and not simply bounce up and down on the spring.
Also pop your head underneath and look at the spring carefully It should be easy to see if it has snapped, this would also possibly cause the car to sit lower on one side than the other.
Hope this helps
By the way rear dampers are quite easy to change yourself, so dont get ripped off by a 'fast fit' tyre type place.