I'm going to take a shot in the dark, and ask you if you have a body lift installed.
Do you have a body lift installed?
I have a 3" body lift on my truck, and I had to adjust the linkage as far out as I could, and still had to grind the bolt holes in the shift gate to keep mine in 4 hi. Mine was behaving the exact same way.
Just something for you to look at before you rip the transfer case out.