1994 E320 Estate Self Leveling Suspension Troubles

[little_e]

My 1994 E320 Estate has about 181500 miles on the clock. It's not a Sportline, but it has the sportline suspension (option code 653).

When the car sits unladen, it looks okay. It's not one of those old w124 cars that looks jacked up in the rear, nor does it look like it has a rhino in the trunk.

When I drive the car over a small bump in the road the rear suspension feels very firm, just like my Audi with sport suspension and Bilstein B8's.
...but when I drive over a larger bump, a speedbump or drive fast over speedbumps (like 27-28 mph) it feels like the rear suspension is like.. a sponge.

For the record, there's no funny noises, clunks, clanks, bangs and dings from either the front or rear suspension. I haven't lost any hydraulic oil.

What's most likely to be on the blink? Suspension spheres or the self leveling shocks themselves?