Frame measurements. If your trying to correct thrust angle, you need to know the suspension mounting points are square to each other and to the body, otherwise your trying to shim the rear to match the front, which either end could have some sway going on that is not readily apparent to the eye. If everything outback is square to the cabin and the front is swayed you would introduce dog tracking by shimming the rear, but damn if you can't get pretty decent alignment #'s.
