Hi Louie,

I assume you have the transmission in neutral and your foot on the brakes when you try to switch it? Lots of people don't realize that has to be done.
If you're doing that right I would have someone sit inside the truck with their foot on the brake, transmission in neutral while you tap gently on the motor on the transfer case. Sometimes the contacts inside the motor get dirty or whatever the motor won't turn to shift the transfer case.