420 SEL (W126) rear end check

I don't really have a major problem. I suppose its just that my rear end (its an '88 w/ 166K) is just a touch too busy for me. Also, I think I have probably either a slightly loose diff. mount and/or deteriorating bushings. I do have new heavy-duty Bilsteins in the rear.

Is there a fairly quick way to check on the health of the differential mount? Also, as a tangential item, does the quality of the differential oil make a difference in its performance?

Thanks.