I had a similar problem last week end, the rear bearing on the center diff would lock up from time to time for no apparent reason!!
It took me a couple of minutes to actualy localize the problem because the bearing would spin freely at times and just lock on it's own...
If you really can't find anything else double check or just change the bearings on the center diff.
when you first dismount the diff just keep spinning the diff in your hand and see if it locks up at some point.