It may be a vent hose, I don’t think it has anything to do with the 4WD however. I believe that system uses an electronic servo to engage the front differential and it may also have one on the transfer case, I think the next step would be to get all 4 wheels up off of the ground and activate the 4WD if the driveshaft going to the front diff doesn’t’ engage then I would suspect an issue in the transfer case, if it does then I would narrow my diagnosis to the front differential.
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