There is a neutral on the transfer case so the truck can be towed.
With the t-case in neutral both driveshafts are disconnected from the t-case.
It goes like this...engine turns the transmission turns the t-case turns the driveshafts turns the differentials turns the axle shafts turns the wheels.
If the engine was off and you towed the truck then everything would still turn but the wheels would turn the axle shafts turn the differentials turn the driveshafts turn the t-case turn the transmission.
The parking brake is on the back of the t-case, it is a drum, by setting the parking brake you lock both driveshafts.
Now lets go through your possible scenario, your front driveshaft broke, you put the transmission in park (but because that only keeps the truck from moving if both driveshafts are connected) you set the parking brake to keep the truck from rolling away while you waited for the tow truck.
The tow truck driver got there, put the transmission in neutral but did not realize the parking brake was set.
As he was winching your truck onto his flatbed the parking brake was trying its hardest to keep the truck from moving but because his 20,000lb winch is to strong something had to give.
So you heard a "POP" as the rear wheels were skidding and then they started moving because something broke.
Could be he blew the rear differential, could be something inside the transfer case, could be a axle shaft, could be the parking brake.
Without being there to inspect the truck I cannot say for sure what broke, but something broke.
You will need to get the truck on a lift and start investigating.