w123 240td lopsided

[sceh]

The car has self levelling suspension which works fine and I have changed the spheres. It appear to a little lopsided at the rear. It has 75k miles from new so it is not really worn and has not had heavy loads.
So, is this a spring issue, a hydraulic ram issue or is there some other thing that can be adjusted?
Has anyone changed the springs on this car and know if it is difficult and how to do it?

thanks for any help