4x4 question

Looking at picking up an '85 F-150 w/ the 300 inline 6 cyl, 4 spd, 4x4 and I've never messed w/ the 4x4 system. Apparently the 4X4 will engage 4H when you start moving. You can cycle the shifter freely into 2H, 4H, & 4L and you can hold it in 2H while you drive, but it will slip into 4H while driving if not holding it. Any ideas if this is a quick easy fix or something in-depth? I'm sure it could be numerous things, just trying to make a quick decision and any help would be greatly appreciated.

That's not a quick fix.  There are plastic pieces in the transfer case that provide the detents to keep it in the proper gear.  I had a t-case that did something similar to what you are seeing because of those pieces being worn out or broken.

But you have to take the t-case apart to replace them.

Well dammit! Not what I was hoping to hear. Thanks for the information Gary.

Depending on the transfer case, they are cheap and easy to rebuild. (see my Vermont saga a few years back)

If it won't stay in gear then I would concour with Gary. It's the plastic shift forks..