just a follow up...
I have the same situation as you describe, and wanted to follow up with you. Did you ever have the work done? Was your description of "sub frame mounts" accurate when you went to the shop? How much did you pay for the work to be done?
1980 300SD (Anthrcite Gray) 253,108 miles
1982 300SD (Silver Blue) 242,398 miles
1982 300SD (Black) 263,185 miles