If the self leveling suspension (SLS) works, I've never heard of it feeling soft. The more common problem is the nitrogen spheres fail and the suspension is rock hard. Maybe what I read as '60s Lincoln wallowing is actually overly stiff suspension bouncing on the tires. Failure of the struts themselves will leave the wagon dragging its posterior. If you load the cargo area - call a few hefty buddies to sit on the rear bumper - with the engine off, does it rise to level when you run the engine?

Or the conventional front shocks are so worn that the whole car feels soft.

Sixto
83 300SD
98 E320 wagon