That route will work, but if you go to ebay, you can often find complete sets of UE ultralites and 6 spider diffs for about $160- cheaper than buying the cases, then FLM metal cups and modified ring gears probably.
The LST2 / muggy diffs would be much cheaper and almost as strong; all you have to do is dremel a small bit of plastic to make them fit, its not as hard as you imagine it to be....
http://cgi.ebay.com/robinson-racing-...d=p3286.c0.m14 $70 for 2x complete metal case and cup diffs.