Ever figure out what this was? Sounds exactly like a trashed input bearing in the t-case to me. The new chain drive throw-it-in-the-dumpster t-cases of today tend to wipe the cheezball input bearing/bushing when run low on ATF. Under power everything seams normal, but usually heavy decel will push the input forward enough to slip the teeth and you get a sound like you're grinding gears. It always sounds like it's coming from the transmission too. Probably because the sound is transmitted through the mainshaft and into the bellhousing area.