The diff in my old Stang was making some chatter around sharp turns (at 55k miles, so I changed the fluid to synthetic. Quieted it down a lot.
If you rear end ever gets submerged in water (driving through a deep puddle or something), I've heard that can contaminate the fluid and make it burn up. The fluid in my car was all black when I changed it.
Anyway, sounds like your problem is tranny related, as Silver-Bolt said. It isn't a miss in the engine, because of a plug backing itself out is it???