Welcome to 2CarPros.
The two issues you're describing are more than likely unrelated. To simplify your post, I would suggest starting a second thread with your posi-traction question.
As for the four wheel drive issue, does the truck have a manual shift transfer case(lever on the floor) or electronic shift(knob on the dash)? If it's manual shift, I would suggest disconnecting the shift linkage from the lever on the transfer case and attempt shifting it by moving the lever. See the attached picture. The component circled in red is what must be disconnected. This will eliminate any potential binding issues with the linkage. If you're able to shift it by hand and it's still not engaging into 4wd, there's most likely an internal transfer issue.
If it's an electronic shift system, can you describe what the indicator lamp on the dash does when you shift to 4wd? Does it turn on solid or flash continually?
Thursday, October 31st, 2019 AT 7:01 PM