I have a 2001 GMC JIMMY SLS, I was driving threw a parking lot (in 2wd) and I felt my front wheels binding as I turned. I stoped and reached my hand up behind the front bumper so I could feel the actuator under the battery tray. It was activated locking the front axel, so I pulled off the vacume line to disengage it. After that it drove normally.
Later as I was troubleshooting this issue I thought the vacuum switch was stuck open, but it's not it is being commanded open. There is power being aplied to it causing it to stay open and aply vacume to the actuator.

The 4wd switch on the dash shows that it's in 2wd.

What could be causing this?

 

Take a look at the switch on top of the transfer case. It is activated when you press the buttons on the dash, which allows vacuum sent to the actuator. Could have failed somehow, though I've never heard of that happening. Not pricey to replace, but is a PITA unless you drop the transfer case.

 

On my model here is no switch on the tc, It's a nv8, the switch is on the firewall and it is electric controlled by the 4 button switch on the dash, and the tccm.
Thanks for your response.

 

inside passenger kick panel is your case command module. Go to hardware store get a welding tip cleaner, and a baby 1/8 standard screwdriver, amd a toothbrush size brass wire brush. . Pull kick panel off pull tccm, look at your pins and plug you'll see green ****.use the correct size welding tip cleaner to clean the plug. If it's a tight fit it's to big. Should be just tight enough to be able to rub the sides just a little little bit or one size smaller. Smaller is better, if you stretch it out there's no fixing it. I prefer the baby standard screwdriver to clean the pins on the Tcase Control module. Remember treat it like baby skin and you'll be fine. If you don't well you'll be at the pick and pull tomorrow to try again.
That's a very very very bad day though.