Scenario....
Anticipating needing 4wd, I turn the selector to 4wd, proceed forward, and pow, a huge pop from the front end. I proceed about 5 feet, and am stuck in mud without front wheel engagement. I manually lock hubs and have 4wd. Is this the leaking hub vacuum, I hear so much about? What was the popping sound? Any harm done? Everything locks, lights up, then disengages......

A little freaked out!

 

Those stock OEM hubs are unreliable in that respect (sometimes won't disengage when they should and vice versa) as they get older with lots of miles. I eliminated that nonsense by installing a Dynatrac free spin kit with Warn manual hubs. Now I know for sure when the hubs are locked 'cause I lock 'em and unlock 'em manually. The big plus to this conversion is that the infernal sealed unit bearing hubs are completely eliminated as well and instead one has bearings which can be serviced or replaced as necessary without the expense of replacing complete hubs.