I have a 2006 F-350 XLT with the electronic 4x4 engagement. When I turn the switch to 4 high, it does not seem as if the hubs are being vacuum locked. If turned manually, 4x4 works fine. I’ve been on YouTube and trying to read articles about this issue and it seems that a lot of people have this problem. I tried the hvac vent trick to test for a vacuum system leak and the hvac system did not switch to defrost. I’m wondering if my locking hubs are shot? They are both very hard to turn and I have to use pliers every time and I am afraid they will break when I need them most. Has anyone solved this issue before?