Before totally condemning the springs -- check the trailing arm bushings very carefully (can be hard to see). I'd think that with accumulated mileage and then the introduction of heavy loads the rubber bushings would give up before the springs in just a months time. Bad TABS will cause the rear to sag.
You probably do need overload springs though and my experience with a set (unknown source) led me to change out a good set of standard rear shocks for Bilstein HDs on my '80 245. That rode too stiffly for me unloaded and skittish in the wet - left in the HDs and went back to standard springs as a happy compromise for me. - Dave