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?
				
			 
	 
 
		 
	