He wasn't doing any thing that outrageous. Just a little stuck and minor tire spin to ease out of it and a fairly siginificant pop let us know that something was amiss.
I think that the spring loaded shifter let the shift fork move slightly under torque and it sheared off the tips of the teeth that engage each other to lock the axles together. That's based on the small chunks of metal in the bottom of the diff and the fact that the engagement collar will no longer slide over to the engage side. Not even with a good pry with a big screwdriver.