I have some personal experience with your exact situation and have spent several months working the issues out of the sagging rear on the SDL.

The first thing that should be done, before any consideration is given to the springs, is the differential mount. This is the steel and rubber piece that bolts the subframe to the body. It doesn't actually hold the differential, but is located right behind the differential. Over 20 years, the rubber compresses and the ride height will drop by about 3/4".

The second thing that should be done are the subframe bushings, which sit between the body and the subframe. These are located at approximately the middle of the passenger door, very closed to the outside of the vehicle. If you go underneath, you can find them by looking for a single very large bolt. The bolt secures the subframe bushing to the body. You can't see the bushing because it has a sheet metal cover over the top of it. Two additional, smaller screws hold the cover to the body.

Personally, I don't believe the subframe bushings affect the ride height all that much, due to their location, however, after 20 years they get brittle and have cracked in a couple of places, so they are due to go.

After doing this work, then replacement of the springs can be considered, if the ride height is not up to par.