Do a search on W140 rear self-levelling suspension, there are lots of posts covering this issue.

Since your post does not specify your location, I will asume you have an U.S. spec S 500 with rear self-levelling suspension, so basically you should start looking at the accumulators, then at the struts, then the pump. A hydraulic fluid change would also be a good idea.

You do not need to change the front ones when changing the back struts. Hydraulic schocks does not even require to change both of them if one fails, just change the one that leaks. Accumulators, on the other hand need to be changed both.

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A. Rosich
CL 500, 1998
S 500 L, 1998
E 320 T, 1995 [Sadly sold ]