I think the rear hydraulic level control valve may have failed on our 1983 300TD.
It is the original valve, has 352k miles on it.
The back of the car is sagging at all times/loads.
I checked that the linkage is intact on the valve and on the suspensions attachments.
I checked that there is a good volume of oil flow through the filter when running.
The oil is dark, and is was changed w/ filter a few months ago so something has polluted it.
Thus my conclusion.
I searched the forum and found that DIY rebuilds don't seem to last very long.
I checked my usual suppliers and no one sells the p/n "1233200158 rear height control valve"