For anyone looking to do xcase and diffs on the Xdrive, Thebmwpartsstore.com is selling the fluids relatively cheap. Hopefully someone can take advantage of my research which came too late as i already ordered my fluids from Turner
Search by part number
Xcase fluid 83220397244 - $38.76
Differential fluid 83 22 1 467 993 - $9.85 (you need 3 for front/rear)
Shipping - $5.50
The necessary tools (14MM Hex) and transfer pump is cheap as hell at Harbor Freight too. $4.99 promotion going on for the transfer pump. i bought two just for the heck of it.