I've recently done this with my friend for his 2006 530xi. It wasn't so bad but I do have a scissor lift which helped. We had to remove the transfer case support bracket in order to remove the sleeve (and the fill plug of the transfer case as we did both a tranny and a transfer case flush). We've removed the front exhaust hanger bracket, the hanger bracket by the transfer case and then we unbolted the exhaust at the front and we just pushed it back and down. That gave use enough clearance...we also had to remove the heat shield (PITA) but it gets easier when removing the support bracket. Using flex/wobble sockets was a must. We've supported the transfer case using a 2x4 which we cut to length so one end would touch the floor, the other would be supporting the transfer case.
2007 530 xi
2004 X3 3.0i