My 1991 126 had a driveline vibe under acceleration - occurred at about 18 MPH, then went away. Quite a few guys on here have had this problem also, with no fix reported.
It slowly got worse, and I assumed it was the center U-joint, sine I had replaced just about everything else.

Bought a rebuilt driveshaft from the shop in San Antonio, and the problem is fixed.

However, after inspecting the old and new shafts, I have concluded that the real problem was possibly THE BIG CENTER SPLINE NUT on the shaft.

About 15 years ago, I had the dealer replace the rear main seal under warranty - they did a mediocore job at best.
When I went back in about 5 years later, I saw the spline nut was loose, so I tightened it to spec (30-40 Nm) and moved on.
However, next time I replaced a flex joint, I observed I could still slide the spline joint rearward, with the big nut properly tightened.

Apparantly, from years of being loose, the spline joint wore to the point that the nut would not tighten it under normal torque, and it clicked and slid forward-back under load, moving around enough to cause the vibration.
The new shaft tightened right up, so this is what made me suspicious.

May have been the u-joint all along, but checking the spline/nut for tightness is certainly worth a try to those wha have reported this type of vibe.

DG

Re: '88 190E 2.6 5-speed, 82K miles

I've developed what I would describle as a cyclic "squeaking" noise between about 16 and 18 MPH that peaks at 17 MPH and the frequency appears to correspond to driveshaft speed. It sounds like it's coming from somewhere between the center driveshaft bearing and the rear axle. It is really only noticeable during the first mile or so after a morning cold start when I'm driving slowly through the neighborhood streets with many stop signs. I can feel no vibration - just hear the noise.

Is this what you experienced?

BTW my center driveshaft bearing was replaced under warranty at about 40K miles circa 1991. I recall that it made a low frequency growling sound typical of a spalled bearing. I suggested to the dealer that the center driveshaft bearing was failing. They verified it, replaced the bearing, and the growling went away.

Duke

Duke, sounds like a very similar issue. Mine didn't squeak, probably cause I greased the splines once years ago. Mine just had a little thumping vibe at driveshaft speed. I'll bet they are due to the same problem - the big nut not locking the splines down well enough. I could actually grab one end of the shaft tube and twist it back & forth by hand and hear/feel the splines click on each other. By the way, I found a big crescent wrench at Harbor Freight that just fits the 46mm nut.

DG

Thanks! I have to replace the shift linkage bushings at the shifter end in the near future, so when I go under for that job, I'll check the driveshaft nut.

Duke

The driveshaft collet nut was tight, but I found the problem, and I'll start a new thread on it.

At least the new shift linkage bushings firmed up the sloppy shifter, and it's an easy job.

Duke