Looking at an '84 300TD tomorrow... Advice?
 

Mainly, I'm interested in figuring out if there's an easy way to diagnose the rear suspension sagging problem. Is there a way of determining which component is out of whack without disassembling the entire thing. The car I'm looking at has a sagging rear (made moreso by the current owner carrying tools in the rear)... Any advice?

Best thing for you would be to do a search. Try words like 'self levelling suspension'.

First things to go are the big black spheres beside the hydraulic shocks. Then the shocks themselves. Finally, the central hydraulic oil regulator and/or pump (first one located on the rear axle, last one on the engine compartment).

When the spheres are gone, usually the car will keep the original riding level but would ride very hard and bumpy. When the rear end sags, then most likely one or more of the other items are gone too. Very expensive $$$ to fix.