So you know now the problem is with the axle end of the driveshaft out to the wheel now what exactly I can't really say seems like you have checked everything. If you had another axle end part of the drive shaft I would say try changing it out. I had a problem with a Traxxas drive shaft where the axle end spun and came out don't know if that's the case with yours the axle end could be spinning loose inside. I don't think I can be of much more help seems like you checked everything I can think of.
I think I broke one of the pieces in the axle, perhaps behind the part that the wheel attaches too. Taking it to the LHS this weekend to have them help further diagnose it, now that I at least know what could possibly be wrong with it.
It looks like you are possible binding inside your tranny, or your spur gear is slipping. Try this, spin your wheels by hand and see if they spin, and then unscrew the screws that hold your transmission together, like halfway out. Try spinning wheels again and compare. At this point if screws are provide resistance, and still spinning, is because you have broke then lose. If after loosening screws helps, then they are installed wrong and too long. Put shorter screws in their place, and you should be good.
Spur gear, yes gears probably look good, but with so much power by the looks of things, your just spinning and burning then pads. Try cranking your spur gear nut at the end of the spring/shaft all the way tight. See if this helps. I am leaning towards the first option.
Otherwise, if niether of these work, then sad to say something is wrong with your tranny gears.