thought I would chime in on the subject as my '95 W124 wagon has had the same low frequency thromb (I made that word up) resonance for a few years. I have had the tires replaced, balanced, rotated, & now that I live in Canada installed another set of tires (winter) on an entirely different set of wheels. None of these changes had an effect on the low frequency vibration or resonance that occurs between 40 & 70 mph. I too am stumped. My indie is going to pull the shaft in a couple of weeks and send it to a shop and see if it is balance. The only other change might be the u-joint between the two shaft segments.
I am curious to know how you make out.

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