Today I installed my 4x4 axles and put on my rotors and spindles on the front end. When I spin the "dealy thing" (don't know what it is called

), only the left axle spins. If I spin the right axle by hand, the "dealy thing" doesn't rotate. Why does the left but not the right axle engage when the "dealy thing" is spun? Thanks for the help if you have any idea what I am talking about.