Im not shure where you can go in Ok.
But here's an idea for ya buy a used 700r4 O/D tranny for as cheap as you can find may be 300$
Then buy a prowertrax posi unit there around 300$ and it requires no special tools to install so you can do it and save some $$$ and now you still have 200 bux left for something else
What gears r in your truck now?