Don't worry, you're not the only one.. I broken 3 - 2 at a time yesterday. I finally e-mailed HPI about this last night (included pictures, you should too if you e-mail them), and received a reply this morning.
Matt,
I am sorry to hear you have had so many issues with the diff. gears on your E-Savage. We have seen a few cases of this happening but not an outrageous amount. I can give you a set of gears for both the front and rear end if you would please send the following information: full name, shipping address-no P.O. box, and a phone number incase we have shipping errors. Once I have that I can go ahead and ship them out. They are a slightly different material and should hold up a little better. Thanks!
Aparantly they're slightly different material, so this is good news! I don't want anything to do with the previous ones I broke, so I'll see how these hold up.