Do you have the hubs that are auto and locking type? Hard to explain if you don't know what I'm talking about but at any rate...our fleet trucks have them (3 F250s and 4 F350s) and so far all but one have failed. Some multiple times and ford won't fix it so we just manually lock them when we get on the jobsite. They would pop and bang and push the front end in a turn but the 4wd wasn't engaged. They didn't actually, "break" but our mechanic said something about them slipping inside themselves. IDK how they work so that's about all the help I can be lol. Sorry