How many miles are on the car?

Manny, the driveshaft is 2 pieces with a u-joint in the center. The joint is staked into position and deamed "replace as a unit" by mercedes.

Some have commmented on W124's to adding a washer between the brg and frame to change the driveshaft angle slightly as a cure to a slightly worn brg.

Driveshaft to me should be directly porportional to speed. IT should continue to increase way past 72 mph. You'd notice it even more at say 100. Like I said- porportional. How about a bad rear brg. Have you rotated the tires front to back. See if the problem magically appears on the front?
Check wheel brg play in all 4 wheels by muscling the tires with the wheel off the ground.

There are a handful of driveshaft shops that can replace the brg for much less than the price of a new driveshaft assy- if it comes to that.
Michael

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Michael McGuire
83 300d
01 vw A4 TDI
66 Chevy Corsa
68 GMC V6 w/oD
86 300E