A hard drop could be your problem, I cant honestly claim that there is no way you could damage it by dropping it or mishandling it.

Drying out isn't an issue, or every time you let the car sit you would be replacing rear ends. The pinion bearings and some of the other parts inside that diff aren't constantly submerged in oil. Instead they get lubricated when the internals are moving.

I cant say exactly how the oil gets slung around inside the monty diff, but usually the ring slings oil onto the pinion bearings and so on. So as soon as you got it moving again, it would be receiving all the oil it needs.

That is assuming of course that the shop put oil back in the diff after they were done with it.