To eliminate rear end sag you must change your Differential Mount AND Subframe Bushings (the two very large round ones).

This is going to sound snooty but forgive me:

The w126 subframe architecture is such that the diff mount and subframe bushings work IN CONJUNCTION with eachother to create a positive-camber platform. If any of those 3 rubber bits weakens or fails, the leveraging effect of the system fails, you lose camber, undue force is put on the springs, and the car saggs.

Also, asymetrical rear end sagging is almost always due to asymetry within the system (i.e. One subframe bushing is bad)

Only after this subframe system fails does excessive pressure begin to be placed on the shocks and springs, leading to their loss of strength, thus exaserbating the sagging further.