4X4 fuse keeps blowing?

Plug in a temporary test bulb in place of whichever fuse is blowing, then you can work on the circuit while it's energized. The bulb will limit current to a safe one amp. You need to poke, wiggle, or bounce on everything you can see until that test bulb becomes bright, indicating the short is occurring. At that point move wire harnesses around and unplug things until the short is out of the circuit. That is when the test bulb will get dim or go out. You could find corrosion between two adjacent terminals in a connector, a mud flap screw run through a wiring harness, a harness that fell down onto hot exhaust parts, a shorted motor, a shorted module, or more commonly, a harness that has been sliding back and forth as the engine rocks, and one of the wires is rubbed through and is touching something metal.

GM was also famous for running the fuel pump wire in a harness right under the driver's right foot area under the carpet on their front-wheel-drive cars. That was a common place to find wires shorted to the body. You might look in that area of your truck too.

Let me take a run at it. Here is the wiring diagrams so we can see how the system works. I would unplug the transfer case shift control switch. And replace the fuse to see if it blows. If not replace the switch. Check out the diagrams (below). Please let us know what you find. We are interested to see what it is.