Might check the fluid level in the transfer case. Remember, if you need to add fluid the newer cases
Do Not use ATF. Most of them use a GM only fluid.
Also check for any loose connections to the actuator motor on the front axle and to the transfer case shifting motor.
When you push a 4WD button do you hear any noise like something might be trying to work?
Just some simple things you might check before going to the shop. Not that the shop is a bad place to go. It's just a little embarrassing to find out the problem is only a loose wire.
