You could upgrade the diffs to BP gears without changing anything else...
http://www.tcscrawlers.com/HPI-Racin...2-p-17874.html

I'd at least upgrade to 4 gear spiders and metal diff cup as well.
http://www.hpiracing.com/products/en/87193/
http://www.hpiracing.com/products/en/86827/

You definitely don't need new arms to install BP gears lol, if you want to drop in prebuilt, complete, BP's they come with the HD larger cups so either swap your cups in there or you need XL dogbones and outer axles with the spacer, this will also make your truck wider like an XL.

BP's will drop right into the 25 housing.

Or you could find complete Flux assemblies on ebay, and add the dogbones and outer axles.
You will need to change a few other things to do that way, I forget what ATM.
(Bumpers and braces I think)