I think the rear suspension may well be "tired.." I agree, the suspension can probably not be improved.. That 4500 pound monster handles surprisingly well, even at her advanced age..

I've had air shocks on the back of my Ranchero since the mid 90's, replacing a set that failed. Yup, they are harsh, "aired up..."

My logic was that, in doing that-- I wouldn't have to affect it much, until I needed the additional "lift.."

Wish it was a dual shock setup, or there was a way to do "accessory air bags.."

I've yet to see a w126 that didn't look "low" in the back.. Not sure what the "stock" ride height was/is.. I'd considered the spring helpers.. I guess I could "wrench those up" when needed.

The shocks need replaced, I'm certain, just wondering what anyone else has done..

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Agnes (the Limo) '84 300SD
Pearl (the Taxi) '81 240D (preparing for the 'afterlife')
Bridgette (baby girl) '82 VW Pickup-- currently getting readied for her rebuilt (overbored) engine.
1 VW 1.6 NA diesel **not installed in vehicle**
Gidget (the bimbo) '87 VW Cabriolet-- the diesel engine it above is eventually going there!

and a BMW Motorcycle. (1998 R1200C)

Did I mention I'm of German Heritage??