I'm not sure if this is "taboo", but here is basically what Linc is talking about it.. only with pix. It's from the LSTForum, and specifically how to shim an LST2 diff. The XXL diff's are exactly the same except I believe they have the better HD cup in them and use a slightly beefier ring and pinion gear. But I can't confirm that.
http://hdrcvideos.com/forums/viewtopic.php?f=18&t=15378
ERBE: RCM 1/8 Hybrid Diffs, NEU 1515/2.5d/S MMM, 6S,
Losi E-XXL CUSTOM, Neu1515 2.5d, MMM, 6s
E-SLAYVO PRO ERevo Chassis w/ Pro 3.3 parts, NEU 1512, MMM, 4S.