I did that job a couple of years ago including the differential mount and ended up with negative toe on the rear wheels.
The rear wheels are mounted on trailing arms attached to the subframe which is eliptically shaped. As a result, when the wheel goes up the toe goes out. When the mounts wear down and compress you get rear wheels that are toed out so the car wanders.
In my case, the subframe and differential mounts helped a lot but I still ended up with the rear wheels toed out. To fix the problem I installed new 19 mm rubber spring shims. (L5007-72928 $6.44 each in Fastlane).
What happens is that after 20 years the rubber gets hard and compresses just like the mounts so your alignment gets messed up. All that new rubber straightens out the alignment and cushions everything so you get an amazing new car ride.
With a spring compressor you can replace the spring shims in an hour. If that doesn't work then the only thing left is the trailing arm bushings and that's a tough job.

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Tom Hughes
St. Louis
84 300SD
92 300D
86 300SDL