My dad's truck got stuck in low range once, it turned out that his shifter boot had gone away letting water into the top of the case and rusted up the shift rails pretty good. We freed it up and it worked fine for a few years till we made him a new case out of a few spare cases....
Pull the shifter boots off and take the shifter out and tell us if anything looks wrong, or snap a pic if you're not sure.
Welcome to Mud!!!!!!

The first advice is free, then we want pics of your truck.....