Update on a previous post, pulled the transfer case and extension housing, the large 5th gear is missing teeth as I suspected.
All the other gears felt fine.
I have a spare NV4500 (unfortunately 2 wheel drive main shaft) that I guess I can pull the two gears from, but don't want to hurt any more parts until I figure out the cause.
Thoughts? The nut backed off a couple of times before I put in the permanent fix, so maybe the small gear riding on the edge of the big gear weakened the teeth on the edge?
http://i112.photobucket.com/albums/n192 ... vkoyft.jpg