Rear end noise, see video.

Need help with rear end noise, I can hear a "thunk" type noise when I take off from a stop.....not all the time but enough to be annoying. Sounds like a bad U-joint but the U-joints check out good. I opened up the diff. to look inside, check out my video. I am not sure how much play there should be when moving the rear wheels back and forth....seems a bit much to me. I am pretty sure the noise in the video is the same I hear when driving. Any ideas?

video link: http://smg.photobucket.com/albums/v669/krtw/?action=view&current=MVI_1335.flv