I jacked up the rear and my tire spun around in circles both ways and I assume it shouldn't. I know the fronts aren't supposed to and they don't but what about the rear?

 

The rears aren't attached to the drivetrain. They will spin in any direction. The front ones ARE a part of the drivetrain, which is why they don't spin freely like the rears. I have some vague recollection as to why they only spin a certain way but I don't want to embarrass myself trying to explain it.