The ZF Nivomat load levelling shocks have been used in the rear suspension of Chrysler vehicles for several years now. According to this article, the shocks will adjust to level the vehicle after about a mile to a mile-and-a-half of driving.The tow package adds the load leveling suspension too (a special ZF shock) so you won't have to worry about loading it down. I've noticed some sit higher at the rear too, but then I go back and look at the same vehicle the next day and it sits lower. Perhaps the load-leveling shock has some variability? It actually doesn't do any leveling until you drive for a short distance (no info on how far) for the internal shock pumping to adjust the rear ride height to "normal".