Check the steering stabilizer, the horizontal "shock." Just drop a bolt form one end and see if it has any compression. My SD does the same thing from time-to-time and the stabilizer is original at 240k (on a long list of replacement items).

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Rick
Avid Benz watcher

Previously owned 126's: 2 1981 300SD's, 1987 300SDL, 1991 350SDL,
Previously owned 123's: 1978 300D, 1983 240D, 1985 300D
Previously owned 124: 1994 320TE

Currently:
2004 Toyota Sequoia SR5 206k
2010 Ford Edge 122k
Always looking for the next MB diesel!