Well at least I'm not the only 126 owner having this problem. On my '91 300SE I have replaced steer shock, lower joints, L/R shocks, tie rods, idler arm, strut rod bushings, 2 alignments and numerous balances. I can now get up to about 65-70 before vibration gets really noticeable, the car does more wandering than what I like instead of pulling to one side or the other like it did when I first bought it. I usually don't have to travel the freeway so when my present Goodyears wear out I'll try an alignment at the dealer in Macon after checking the center link again.

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1991 300SE (my ride, 279,000 miles, looks almost new
1954 Cadillac (21 yo son's car, he bought when age 15)
1972 SeaBird 19 ft runabout (old but solid, slant six, Volvo sterndrive perfect condition, undergoing complete overhaul and refit)
1998 Toyota Rav4 (my sons daily driver when he is in the Continental US, PROUDLY serving in US Navy)