560SEC Left Side lower than Right Side
 

Hi. My 1988 560SEC with 33K miles is 1/2 inch lower at the rear on the driver's side than on the rear passenger side and 1/4 inch lower at the front driver's side than on the front passenger side. The difference is enough to notice, as if the car had a flat rear tire. I assume the problem must be with the rear self-leveling system since sometimes the car stays level. Any ideas?
Thanks.