My truck has the auto hubs and it has this weird problem where it can no longer shift from 2HI to 4HI while driving under 25 mph. If I attempt to do that, there is a grinding sound so I don't do it. However, if I stop and put it into P or N, I can shift from 2HI to 4HI. Then if I put it in D and drive forward to lock the hubs, the 4x4 does work. Once the hubs have been locked, everything works again, I can shift between 2H and 4H on the fly under 25 mph unless I leave it in 2H and reverse, which unlocks the hubs.
The transfer case seems to work good and it has fresh oil. The only problem I can see is something with the automatic hubs or something else in those spots.