W126 - Swimmy rear-end on Hwy driving

[ck42]

Looking for opinions on what may be causing this OR if this 'behavior' is considered normal for my 126.

During hwy driving, I find myself having to make frequent corrections (slight left or right) to drive straight AFTER making a previous correction. Did I lose you yet?

The things is, after I make a correction, it feels like the rear of the car trails the correction by about 1/4 second. This delayed rearend behavior then causes the correction I made to be too much....in which case I have to steer the opposite direction to 'correct' my correction.....as you might imagine, this behavior could then continue indefinitely....each correction followed by another correction.

So....I've developed a driving style that entails making VERY minor and controlled corrections in order to minimize the delayed rearend response.

The rear shocks are originals but appear to be in good shape (no leaking, no bouncing)
All rubber in the rear is also original.

Does this sound like a shock 'thing' or subframe mounts or something else?