Are you sure that your car doesn't have a self-leveling rear suspension? My 1983 300TD wagon used to do the exact same thing; the rear end would raise up in reverse, drop in drive. If your car does have the self-leveling system, it could be an easy fix. Check for a hydraulic fluid tank under the hood. Mine is located in the front left of the engine compartment. You might just be low on hydro fluid. If that's not it, you can adjust the linkage in the rear of the car coming out of the control valve. Disconnect the linkage, and lift up. If it's like my car was, you'll watch the whole rear end of your car head for the sky. Well, that's my two cents. Of course, that's assuming you have a self leveling system.

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1984 300TD Wagon--Astral Silver--Retired
1983 300TD Wagon--Dark Blue--Retired