Date registered: May 2013
Vehicle: 1961 190SL
Location: Northern California, USA
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Are you able to take it off yourself and inspect it? I personally don't trust "professionl" mechanics because I have seen the messes they make.
I have not personally seen a MB 2-piece driveshaft, but one of my domestic vehicles, a Dodge van, has one. I have personally replaced the U-joints and center support bearing and here are some of the things that could have been done wrong:
1. Driveshaft halves not put back together in the proper phasing.
2. Needles in U-joint cross displaced if U-joint was handled roughly during cross end cap installation into yoke(s) and/or hammered together. If even one of those needles is dislodged the cap cannot be fully installed. Those caps have to be pressed apart and together, no hammering. I don't know what the "professionals" use but I use a large C-clamp and that works well.
3. Snap rings that retain cross end caps not correctly seated in grooves in yoke(s).
4. Yoke(s) deformed by excessive force in removing/installing cross end caps. When the caps are pressed out, the side of the yoke where the cap is being pressed from must be supported or it will be damaged.
Driveshafts and universal joints are extremely durable but precision parts however careful assembly is required. My bet is that if you remove it and carefully inspect the U-joint phasing and cross end cap and retaining snap ring installation you will see the problem.