Car set too high over all tires, not smooth over small bump.
Are you getting airmatic error messages on your dashboard?
Going to need more details...
Check the level sensors at the two front struts and one above the rear axle. The one at the rear axle can sometimes break. Another member had this issue a while ago. Learn more about it here: https://www.youcanic.com/u/channelzer0/2007-s550-4matic-rear-end-dropped