Regarding the sls: I just rebuilt mine. It may not have been absolutely necessary, though, because once taken apart it was obvious that both the lever on the sway bar and the lever on the valve were loose. If there are no obvious leaks, check those. My sagging rear end was cured with new subframe and differential mounts. Now, if really loaded down, the sls raises the rear back up to level.

‘85 300TD. 398,000 mile daily driver