If the suspension bushings, ball joints, tierod ends, etc are worn out, there's no alignment in the world that's going to make it drive straight.

Sagging springs in the rear on the LWB 126 seems to be common. I just replaced the ones in the SDL and the difference is nothing short of amazing.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]