I have been looking for a diff clunk. I have put in a isolator bushing at the top of the diff nose and still hear the noise (you can see it in the video)
Recently, I went under the car and found the drive shaft had some play at the diff end, not at the U-joint though. See the video.
Where can this slop be from? Is this normal? The diff gear mesh?
How can I fix this? 
Thank you.

Do the output shafts move when you do that? If they don't move, that is inside the diff. It could be re-shimmed but might not last or it could whine once it's re-shimmed

I don't believe so because the front wheels were on ramps and the rear was on the ground.
Actually, the output shafts can still move if that play occurs in the diff. Right?

If there is slack in the output U-joints then the slack may be in the output shafts and not internal to the diff