Hey look I don’t trust dealerships
I had one tell me my factory lockers where installed aftermarket and they can’t fix it.
it was just a seized actuator. Fixed it myself and regret the $300 diagnose fee
id get a compression test done and a blackstone analysis report to see if there is coolant...
There is a lisle funnel you use to burp air out. Park or jack the front up. Put the funnel on and allow the truck to run and give it gas while adding coolant. I’m over simplifying this so please do a deeper dive