Could be that the 2 piece driveshaft was put back together improperly. With these cars, and some other cars (volvo 240 comes to mind) you must mark the 2 pieces of the driveshaft before disassembly, if you don't then you'll have a large chance of reconnecting it in different splines, and the balance will be off just enough to bother you. Same thing happened with my first w123 when I did a motor swap back in 2009. I was just talking to a local mechanic about the same thing last week, and he's had it happen twice on customer cars as well.

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1981 300TD 4 speed manual
Euro bumpers, zender valance and skirts, H&R springs, billy HD's, leveled sls, real AMG Pentas 16x8 et11, vdo boost/egt gauges intergrated into ash tray, eurolights, led 3rd brake light