"service 4WD system" message

This is due to a number of things, so we need to start with checking for codes and finding out what is there.

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

Based on the codes we can make a better decision but the normal cause of this is the final drive control module is not able to shift the transfer case and it is not able to do it.

So, we need to check the voltage coming from the DTCM and find out if when it gets the switch input to shift that it is actually commanding the motor to come on.

Here is a guide that will help with this:

https://www.2carpros.com/articles/how-to-check-wiring

Below you will find the wiring diagram that we need to use to test this.

Please let us know what questions you have, and we can go from there. Thanks