Sorry not a fan of a pinto rack on a mopar. I have done a few for customers with the same results as you have, The rack is so short that the pivot points of the inner joint makes the rod real long giving it a lot of leverage. So when backing up it pulls the toe way in pulling on suspension sucking it in. I bett if you measure your front end height i bet the car actually is raised up higher after backing up so that it changes camber. I would also bet you have a lot of toe change when front end comes up. i would either find a longer rack or or go back to stock. This is not an easy task to chase. Jake