Given drivetrain noises that eluded diagnosis so far from multiple shops, I was thinking a new (used) diff might solve the problem. It looks like a few hundred on ebay will buy one. I'm thinking install is about 2 hours?

May as well take this opportunity to shorten of the final drive ratio a bit. I've seen some threads about complications about with non-M e46 diffs relating to drive shaft length, 4 vs. 6 bolt flanges, etc. What e46 diffs are shorter ratio vs. my stock ZHP and truly plug and play?