I think I've got the gear thing figured out somewhat.
I have a habit, if I'm against a board, of tapping the throttle and letting off real quick. With the low cogging of brushless motors, I can usually marshal myself since the car will bounce and roll backwards. Having two diffs in the car makes this safe, since the diffs will slip and absorb the torque.
So, now I'm running a one way, and the front is essentially locked. Tapping the throttle, especially with all the torque of a brushless motor goes right into the spur. Presumably, the PRS adapter/spurs are more flexible, or maybe because they're farther out or something, but I bet the torque is causing them to bend and strip. That might explain why it didn't happen with foam tires, since I was always running two diffs.
Anyway, that's probably common sense, but I didn't really put together the pieces before now.