need help for my Inferno-10 EP
 

I've built an electric 4WD using the gearboxes and gears of an inferno-10. Somehow the diff ring gear does not seem flat (it's cast metal from a later model). So I had to shim the diff to stop it from skipping teeth against the beveled drive gear. Now it doesn't skip under load anymore, but has more friction than a tank :(
Are any newer gears compatible,or are the plastic molded diffcases with integrated ring gear of the rampage any better?