Very late reply to this, but looking at that broken shaft, I think there may have been a defect in that axle. notice the dark area? That suggests that damage had been there for a while. I've got a 31 spline 8.8 in my '96 Bronco on 36's and the axles survived me blowing up the trac-loc diff into 10 different pieces.
(351W, E4OD in low, tcase in low, tires aired down to 10, going up a ledge.)