Only one front wheel engages when in 4x4, no power to front wheels

What is your question?

I have confirmed with both tires off the ground that only one of the front wheels engages in 4x4, and since it is an open differential, no power gets transmitted to the front wheels. Could this be a vacuum issue? Or is the hub a more likely culprit.

What makes this problem better or worse?

Nothing. Same wheel does not engage (passenger side)

How long has your 2008 Ford F-350 Super Duty had this problem?

Over a month