So, here's the latest entry to this little saga.
When I talked to my service manager on monday, he asked me to bring it in again, and they'd take a look. The last thing he mentioned was calling mazda to see what they thought, and if they would make me go back to stock.
So I rolled in today, all ready to do battle--had all my arguments together for how my mods could not possibly be causing the noise... rasied the pipe way off the crossmember, shoved a chunk of coolant hose between the pipe and crossmember, sound persists even while coasting with the engine off and tranny in neutral... everything. And as a last ditch effort, I was ready to try and convince them to let me at least leave the rear mount in, because it's a bitch and a half to replace.
Anyways, after watching CNN talk about the Dow Jones plunging 300 points today (OMG! SELL SELL SELL!!!!!) for half an hour, he comes upstairs and lets me know that they want to take apart the differential first thing next week. Apparently mazda's people agreed with me that it was at least unlikely enough that my mods were the problem that they're willing to authorize having my diff inspected.
It appears that things are looking up for me... I'm just glad I've been keeping persistent but polite about this whole thing. I'm also returning to my original highly positive opinion of Morries' service department. The service manager I've been working with has been a lot more accommodating than I would expect from a dealer (even if they frustrated me a bit with their focus on the exhaust pipe). So, best case scenario I'm looking at a brand new LSD and a week or so in a loaner.... but we'll see what happens.