The bevel pinions in the differential normally turn very slowly compared to the rest of the gears, so they probably have plain bronze journal bearings with very little clearance. If they're forced to turn a lot and faster than normal it'll wear those journal bearings. If it spins enough, the pinion shafts could expand and seize in the bearings...although this is a worst case scenario.
If you chock the wheel on the ground, you'll get away with spinning the free wheel for a while..probably longer than a while, but the bevel pinion bearings will wear and you'll have a weaker differential.
Either way, not a good idea, rotating machinery is not forgiving, no matter what part of your body it gets a hold of..if you're willing to take your life in your hands, jack up both rear wheels when doing this. Then at least the car won't really be hurt.
