Look for my post on W126 rear subframe mounts. Essentially my rule of thumb is that there should be a notable and even gap between the subframe carrier and the plate that holds it in place. If the gap is only measurable with a feeler gauge or is uneven, you need new subframe mounts. Someone posted pictures of a good and bad diff mount. For the most part you don't want the center section resting on the bottom of the mount. You want to see a distinct bump either in the bottom of the center section or the part of frame the center section sits on when compressed. I forget where the bump should be exactly.

Sixto
95 S420
87 300SDL
83 300SD