Bad diff?

[chris1110]

so i was use my truck putting on the washer to stop the wheel wobble and when i tried to unscrew the nut on the left in the rear it was make a weird noise almost if the diff was skipping a gear. so i ripped off the nut and tried again. before i put another nut on i looked at every thing, it looked normal. and it still happened. here is a video of the noise.
http://img268.imageshack.us/i/jrl.mp4/
any ideas on what is wrong? i can get pic of the diff if you want. also every is screwed on tightly.