Welcome to Just Answer Dodge my name is XXXXX XXXXX X will be assisting you today.
On the passenger side of the front axle there is a shift motor that will engage the front axles. This motor is vacuum operated, so check and see if you have vacuum getting to this motor with the engine running and in 4wd.
If there is no vacuum, then trace the lines back to the vacuum switch located on top of the transfer case and see if you have vacuum there.
Sometimes these switches stick and need to be replaced, and other times, there is a lack of vacuum getting to the switch and you have to trace the lines and repair the leak.
Let me know if you need anything else.........alright!
That's maybe why my ac/heat vents will only open on defrost.
Yes..........big clue there. So that means you have a loss of vacuum, since the a/c heat controls are vacuum operated. A common area to see problems is below the battery tray where battery acid can leak onto the vacuum line going to the cruise servo and cause a leak.
If that area checks out fine, then just trace the lines until you find a leak or break.
Thanks for your help, will look at this tomorrow.
You're most welcome.
Please don't neglect to provide a positive rating today, and if need be you can always return and ask follow ups as needed.......thx.