Hello all,
Thank you all for the suggestions. It may be something in the steering (except I have recirculating ball type rather than rack and pinion, but there might still be something suspect in there). My 2 front shocks are leaking, so they're probably due for replacement.

Next chance I'll get to tear into it will be the weekend (or if my blood is boiling enough, I may end up finding an indy suspension shop that works on MBZ before that).

Thank you all again for the help and suggestions. I will update this post with what I find out (hopefully the solution!).

SRS

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1996 Mercedes Benz S420 148K miles
1999 Volvo V70 Wagon 120K miles
1988 Chevrolet K1500 4x4 198K miles