You can buy the X2 diffs; but keep in mind you'll need to have the ring gear/pinions as well as the X2's are sprial cut and your X1's may be straight cut if it's an older model.
As far as going lighter...
All the nuts on the suspension components are steel. Go to aluminum. Ofna makes all kinds of colors if you like that.
Lunsford Titanium turnbuckles.
Use a buggy front bumper instead of the truck setup.
Run JConcepts Rulux wheels (half-ups)...they're a bit lighter than any of the dish wheels and you can run JC, PL or AKA tires on 'em. (though the new PL's that are coming will use a different bead)
Do a search for a thread by me in the f/s section; I have a pair of CF shock towers that will fit it. Should knock off around 20+ grams; I'll have to reweigh them for sure.
If you're racing, get a personal transponder instead of a handout (if you're doing that) as the PT is about 1/2 the weight of the traditional h.o. units.
Me personally..I'd be looking for an X2 roller.