Might want to verify that vacuum switches as it should from two, to four wheel drive first..... you can do that in the driveway, having a friend help you makes it easier

Thought maybe you wanted me to jack it up and watch wheels didnt really know what you were talking about in this sentence.
sorry.

 

Nah, just pull the vacuum connector off of the actuator on the front axle, start the truck in 2wd, see which line you have vacuum at. Shift to 4wd, and see if vacuum is now at the OTHER port on that connector... Quick, reasonably painless, and easy.

 

Kind of like the link provided explains.