Odds are that, as suggested above, the transfer case shifter linkage has simply slipped out of adjustment. A bolt on the linkage is supposed to hold it in adjustment but if the bolt isn't tight enough, the linkage can slip out of adjustment so it won't pull the transfer case all the way into gear.
Here's a pic of the linkage from underneath with that bolt circled in red. It's difficult to get to without dropping the skidplate. Be sure to remove those four little 1/2" nuts in the center of the skidplate that go to the transmission mount before removing the six big bolts that hold it to the frame. Support the transmission with a jack or jack stand in front of the skidplate before dropping the skidplate.
Loosening that bolt so you can pull the linkage back into adjustment is the thing to do, it's not hard to do. Tighten the snot out of that bolt once you get the linkage back into adjustment.