If you have anything bent in the rear the alignment shop should have let you know about it.

It sounds like you're dealing with a tire pull - the only fix is to keep trying different combinations of tires on the front end until you find a pair that drives straight. By the time you are ready to rotate your tires again in 7500 miles, your rear tires will hopefully have evened out their wear patterns - assuming you have them correctly inflated and don't have any problems like worn shocks that would cause them to wear unevenly.

Either that or get some new tires. <img src="/forums/images/graemlins/smile.gif" alt="" />