Not sure if this applies to your vehicle but the W126 body 560SEL had self leveling and BY FAR the most common failure was the nitrogen-filled accumulators. The shocks themselves are purported to be very robust. Replacing the accumulators is said to be easy and not too expensive.
If you haven't already done so, do a search on "self leveling" and/or "accumulators" (at least I THINK that's what they're called!) I seem to remember a decent writeup on replacing them somewhere on the site.