Rear End Problems

[72BlckButy]

I was trying to determine what kind of rear end I have, so recently I picked up a new rear end cover, might as well kill two birds with one stone. While proceeding to put it on, I put the rear end of the truck on jack stands. While on jackstands, I spun the right rear wheel.....click.click.click, and the left tire isn't spinning. So I turn it towards the back, click.click.click.....and the left tire kind of spins backwards. I move to the left wheel....turn the wheel forwards....click.click.click....and the right tire spins forwards, as well as turning the wheel backwards.

Any ideas why?

Also, to give a little more information, when I turn corners, I hear clicking out of each turn. I've asked a lot of people and they say that it's probably a posi.