I'm only familiar with the 126. There's a bracket in the shape of a salt shaker that sits under the rear suframe bushings by the jack points. There should be about 1cm clearance all around between the rubber and the bracket. If there's less clearance or uneven clearance, you likely need new subframe bushings.

You should probably replace the diff mount while you're at it. Definitely replace it if the inner piece with two bolts into the diff is sitting on the outer frame.

Sixto
87 300D