Originally Posted by Banksy
I was going to say u-joints, but the fact it isn't engaging the front axle could be a couple things. 1) transfer case or 2) the vacuum actuator in the front axle that locks in the axle when you shift into 4wd. My bet is is on #2 and you want to hope that's it. Check for broken vacuum line to it.
The u-joints could still be junk though.
So after I posted I did a little more research, and I found saw people on various websites talking about the vaccuum actuator. I decided to go crawl under the truck and test it out myself. I swapped out the vaccuum lines and no change. Tommorow I will probably drop down the actuator itself and see if the fork inside is moving around, as that is what engages the front axle