Thanks for the suggestions, especially craigayl' s link. I've also heard that the Olthoff's may have some new & improved mounts.
The more I look at it, the source of my problem seems to be engine block movement. Urethane engine mounts, or some means of strapping / limiting the engine block from it's wild dance (however fetching it may be) sounds like the ultimate cure.
I've been driving the car without rear mounts for a while now. It sure is fun watching the pipes gyrating all over the place, but is probably not doing any favors to my stainless headers....
