The XXL diffs come stock with HD differential cups and HD differential outdrives. They are far superior to the ones that came wwith the LST2 (especially the diff cups). IMO they are just as good, if not better, than the fully metal diff cups you can buy. Shim them properly, and don't do donuts, and they will last a long time (with 40K or less oil in them).
LST2 diff cups LOSB 3537
XXL diff cups LOSB 3528
The main difference is the metal inserts that that insert into the side of the cups which house the pins for the planetary gears. This prevents the the cup from rounding out like this:
This is compared tot he HD cup which looks like this without the metal inserts.
Diagram:
This was a rolling upgrade that Losi did with the XXL so the early ones have regular cups (and outdrives). Thus, when you buy diffs on ebay make sure they come out of a newer XXL for the best possible purcahse.