find someone parting out the new 3905 emaxx on ebay and get the difs from there. I don't think I have heard about anyone blowing the new difs yet. They look to be very much improved over the old difs. Would probably be your cheapest option for getting bullet proof difs. But I cant swear that they are a direct drop in for the 3906 eMaxx. I looks like they should be, but someone here can probably confirm.
EDIT:
Here $22 per dif and it says they are a direct drop in for all Maxx models.