At hwy speed vibration feels like balance problem, re-balance changes nothing, all components are sound but damper was shot so it was replaced; vibration much attenuated but still noticable suggesting that new damper will soon be shot, too.

Can worn coupling assembly cause such problems? Car has 211K miles. Could it need new front wheel bearings?

23 yr. old car with 200K+ miles. The list of suspension/steering parts that could be worn is long. You'll need to find a good shop that specializes in front-end work. Tires could also be an issue. How many miles are on them and are they a quality brand?

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'91 300-SEL

all parts except coupling assembly are tight and in good order; tires have less than 5K on them and are the same brand I have been buying for nearly 30 years with no issues

Does the car pull somewhat hard to the left or right when you let off of the steering wheel on a straight stretch of highway? If so, I would suspect that the front-end needs to be aligned.

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'91 300-SEL

Front alignment was performed last week by (arguably) the best technician in the west. He tweaked a couple of adjustments but found nothing far out of spec. Front sway bar bushings were replaced last year.

NOTE - I have noticed a distinct "klunk" sound coming from left front when traversing even minor irregularities such as entering a parking lot where the driveway is rough.

Ragobro, I have found my '91 300SE to be the most sensitive car to tire balance I have EVER seen. We just went through 3 attempts to balance a new set last month; the tech finally got them right by setting the Hunter machine on "manual" and working them to within less than 1/4 ounce. This happens every time I buy new tires, and I've had her for 240k miles. Once right, they seem to do fine for many miles. The classic symptom is vibration right at 60 MPH, going away at slower or faster speed. Condition of wheel bearings, idler arm, damper, ball joint, bushings, etc; has seemed to make little difference over the years. I've rebuilt everything a few times, and still get balance problems with every new set of tires.
The clunk you hear is quite possibly the guide rod (caster rod) bearing. This is the broomstick-size rod that runs straight back from the bottom of the shock absorber/control arm to the body. The bearing is mounted on the rear at the body attachment subframe. It can be verified by lightly applying the brakes when going over the bump - if the clunk goes away, it's probably the bearing. Do a search, you'll find lots of photos, etc.

DG