I would look at worn out rear control arm bushings if its moving front to back like that, it could be any number of things, when i pulled my control arms off the bushings were so worn they just fell out. looking at the car from the side the wheel was centered, from the rear it was off by a lot to the side.

 

how do you measure your rear end to find out if it is off center and by how much?

 

i could tell by looking at mine it was off, when we loaded it on the trailer it was flat out obvious.
you can measure from basic points on the frame rails or even use the wheel well openings as reference points.