You just need the entire subframe from the M.
http://www.realoem.com/bmw/showparts...33&hg=33&fg=30
http://www.realoem.com/bmw/showparts...93&hg=33&fg=05
The diff carrier is integrated into the rear subframe. If you compare the diagram below for a none-M:
http://www.realoem.com/bmw/showparts...38&hg=33&fg=30
As you can see, the front of the diff on the M bolts onto the subframe from the side (1x) and back (2x). The non-M diff bolts onto the subframe from the side (1x) and the front (2x). I believe you can not swap the internals from the M diff to the non-M diff, since the M diff internals are much larger than the smaller non-M diff.