[bobcowan]

To get a stronger diff, you'll end up with new axles. So count on doing that no matter what you do.

The Ford 8.8 is readily available (in the US), cheap, and heavy duty parts are everywhere. You can't get a 4.27 ratio, but you can get 4.30. Heavy duty axles are available in 28 and 31 spline flavors. You can also get worm gears from Auburn or Torsen.

I don't enough about BMW parts to speak of.