So I got stuck in a deep puddle today. I was in 4x4, but the video taken pulling me out shows the front tires aren't spinning. I felt the transfer case shift into 4x4 and then felt it shift into 4Low, but no tires turning. Would a vacuum leak cause this?

 

It could be weak because of that but you also have a conventional differential with no LS in the front. Worse comes to worse, you could be in 4x4 and only have 1 tire spinning. Even in 4low, there's no guarantee the from tires will spin all the time
PS: we're you mudding outside a school or something?

 

Check the front differential solenoid and the vacuum lines to the axle motor. Make sure the axle fork is engaging. It doesn't seem that front is locked in. You'd have 1 wheel covering your truck in mud right away and both sides would still throw mud in 4L. Just not from both sides at one time.

 

Jimmy John's Sandwiches. I had both rear tires spinning, but no tire movement at all from the front end. We were having a "who's truck's muddier" competition. I won. I got mud inside my bed, and I have a tonneau cover!

 

The fork is a common issue in theses gen trucks

 

Im sure this isnt your problem but i had my 4x4 engagement motor on the trans go bad. That made my truck act all kinds of stupid

 

How can I tell if the fork is engaging?