OK... Lots of thing that you need to describe first before we can help you out.
1) How much oil are you using over what time period? How fast are you going through a quart?
2) Any external leaks or oil build up? If so, where are they located?
3) Any evidence of blow-by? Is there smoke or oil coming out of the breather, dipstick, etc?
4) Have you checked to see if an oil supply or return line off the turbo could be leaking?
5) Have you checked the turbo itself for evidence of bearing or seal failure?
6) Does the truck run OK? Any exhaust smoke, what color, and under what conditions?
Start with these and see if we can narrow it down a little for you.