GMC Troubleshooting Questions? Ask a Mechanic for Answers
If you look at the diagram below it shows where that hose goes to. If i understand you it goes to a vent but not sure.
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There is a fork inside the axle housing that is related to the cable pull. It's what engages the axle. These break all the time. You are going to have to get in there to inspect it and i highly suspect that it is the fault.