I have a 1998 4x4 4.0L extended cab. I was told by a mechanic that the left front vacuum hub was not engaging. However, he was not a very trustworthy mechanic as I came to find (and I didn't give him any of my business) and I'm wondering how I can check this myself. Do I need to get all 4 wheels jacked up and put the truck in 4x4 to check this? If so, what should I be looking for?