Here is the original thread. FWIW I built this spare and then tested it on the car and works fine, no issues with the diff no matter whether I put it on the front or the rear.
http://www.m3post.com/forums/showthread.php?t=253769
Also if you are to lazy to put this thing together yourself you can buy the complete kit from tischer, it's obviously more expensive than buying the pieces separately but they both work the same.