So you haven't touched anything on the rear suspension? You very well could have some worn rubber back there, subframe mounts, diff mount, rear sway bar links, etc. Do a search on "rear steering." I'm not sure what it is exactly but I've heard it mentioned here several times, something about the rear suspension parts being worn and causing the car to want to do interesting things.

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Seth

1984 300D 225K
1985 300D Donor body
1985 300D Turbo 165K. Totaled. Donor Engine. It runs!!!
1980 300SD 311K My New Baby.
1979 BMW 633csi 62K+++? Dead odo