I see that Posi-lok has a pretty good troubleshooting guide to your 4x4 system.
http://www.4x4posi-lok.com/PDFs/TSG_chevyT.pdf
They are of course trying to sell you their system, but that's your choice.
If you get their system look at the pictures closely when you install it, their directions are basicly useless. It should take about 1 hour
I can send you pics of what it looks like in my 85 Blazer if you want.