Indeed, if you upgrade the diff case to metal, then the next weak link is the tiny spider gears inside the traxxas diffs; there are only 4 of them vs 6 or 8 in 1/8 scale diffs. The plastic diff cup itself is also another weak link- do you have the older 3906 emaxx or one of the newer versions with a grey chassis ( 3903/3905/3908 )?
For cheaper 1/8 diff internals there are plenty of resources to look at:
http://www.rc-monster.com/forum/showthread.php?t=8012
http://www.beatyourtruck.com/forum/s...Best-1-8-diffs
http://www.rc-monster.com/forum/showthread.php?t=31594
bearing in mind Ultralites and the FLM Hybrids will use the same internals quite happily with a little shimming.
Really though, if you just want alloy diff cups and cases then look into FLM or GH racing options, but they will not fix the weak spider gear issue. If you want true 1/8 diffs, then either UE or FLM Hybrid bulks ( not the combo bulks ).