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