Well, I'm back to square one again on the tranny. Rennsport Systems kept my transmission for over two months and didn't work on it. I asked the good folks over at 914world.com about the situation and most people suggested that I was waiting far too long and paying far too much for what I was doing. Two folks highly recommended I go out to the Rennsport shop without calling ahead of time and just take the transmission back, just to be sure they didn't start working on it as I drove over.
So that's what I did. Jeff at the shop was gracious enough to give me back the transmission (and the extra-parts tranny) without any questions. They seemed completely stressed out due to the extreme backlog of work, anyway.
I'm in the process of trying to buy a rebuilt tranny from one of the 914world folks, so I hope that pans out. My other option is to have "Dr. Evil" from 914world rebuild a transmission for me for $1500.
My gut feel, not based on any fact, is that a different transmission won't have the same resonant problem as this one does; however, it may come with its own problems. Stay tuned as the saga continues...