There are several good suggestions here.
I also think that the transfer case is "bound up". This is a result of turning while in 4WD. In a turn, the front drive shaft is trying to turn more revolutions than the rear one, and since the two shafts are connected at the transfer case there is an uneven load created at the case. The gears are meshed with a high load on them and they won't release.
Two additional techniques that have worked for me is to drive the truck, preferably on a gravel road (or other slick surface) in a straight line for some distance. This allows the tires to slip enough to take the uneven load off the transfer case.
Another method that I have used is to stop the truck, set the brake, shut it off, and select neutral on the transmission. This will sometimes allow me to get the transfer case to shift, but the other method (driving in a straight line) is more reliable.
Let us know when it's out.
Good luck.
Loren