So, follow up to this thread for any of you all still interested. The new transmission was ready to be installed and during installation they noticed there was a bolt loose somewhere in the driveline. During reverse loads, it was making contact with the heat shield. It was a total simple fix -- tighten the bolt, and the problem was gone. No damage to any component, and my original transmission never had a problem, it just was resonating the noise through there. Dealer had to send back the transmission they ordered for me. I'm glad it wasn't really a serious problem!
