The FLM
HYBRID BULKS are what you need to use 1/8 scale diffs.
You could just use an alloy cup and case from FLM, but that wont fix the fact that stock emaxx diffs are rather shite, with only 4 spider gears, compared to 1/8 diffs that use 6 or 8 spider gears.
The cheapest solution would be the muggy/lst2 diffs which can be bought on ebay for like $20 each; a small amount of work is all thats required to have bullet proof diffs:
http://www.rc-monster.com/forum/showthread.php?t=9153
Much better and cheaper than messing around trying to make stock diffs stronger with an alloy cup and case.
FLM hybrids are similar to the UE VBS setup, or the UE ultralite case which fits any maxx bulks, and uses 1/8 scale inards. You can find a kit in the RCM store with hotbodies 1/8scale buggy diffs and the required pinion gear and bearings etc.