The truck will not shift to four wheel drive

It sounds like the transfer case encoder is out but to be sure lets start by checking of the fuses.

Here is a guide to help us get started:

https://www.2carpros.com/articles/how-to-check-a-car-fuse

Also, lets run the codes.

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

Here are the the fuse panels check all fuses with the key on and how to replace the encoder motor if the fuses are okay. Check out the diagrams (Below). Let us know what happens and please upload pictures or videos of the problem.