Just curious as I am 4wd challenged. If the OP has a locking rear differential and both rear tires are spinning would they turn more slowly than a single tire spinning on the front with the open differential? Another question: are all 4wd trucks equipped with locking differentials?That’s how your open differential works. The tire with the least traction/resistance gets the power. That’s why only one wheel spins but when you stop it with your boot that has more resistance/traction so the power is sent to the other side. A differential locker is the way to get power to both sides equally.
As far as the front spinning faster than the rear I’m at a loss. I suspect that being off the ground will cause some issues in that regard.