[Captainfab]

Have you looked at the trailing arm crossmember to look for signs of movement? A slight bit of movement at the crossmember when extended out the distance of the trailing arm will add up to a fair amount. As already mentioned, I wouldn't be surprised if what you are experiencing was within factory tollerances. It almost sounds like adjustable trailing arms and panhard bar would be the best way to achieve a perfect alignment of the rear end. Yep I know....there's more $$$$