If your rear hydro-pneumatic shocks are not leaking then the culprit would be the nitro-cells. Replacing these would be alot cheaper than going a full conversion. From what I have read, if your rear wheels appear to be "squating", this is caused by the rear suspension bushings being worn out.
1994 MB E320 Coupe (199,500 mi)
1992 MB 190E 2.6 (173,000 mi)
1983 Porsche 911SC (148,000 mi)
2004 Volvo V70 2.5T (198,000 mi)
President of CT/Westchester Section