Vibrations always start with the rotating member; tires wheels etc. These vibrations get out of hand with excessive play in the steering components; tierods, idler arm, and center link. The steering shock usually is weak.
On many models and especially the 126 and 140 models longitudinal softness or play is a recognised deficiency. When this occurs the wheel can move forward and backward in the wheel well. The device used to control this activity is a ball joint/mount in the 126 body and a hydraulic control arm bushing in 140 chassis. Both chassis have characteristic resonant vibrations that occur above and around 60mph due to this softness. Modified improved parts are now sold for both models. These actually were put into production early in both chassis runs. Even the newer parts fail to control this quite early in life. Except for the loss of fluid in the 140 hydraulic bushing and the cracked boots on the 126 ball joint these control pieces are very hard to prove defficient as their original design was marginal (proven by the subsequent improved designs).
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician