I think you mean the truck is 4 Wheel drive.. AWD and 4WD are different in how they operate.
On a AWD vehicle yes, all 4 tires should have the same trad depth, so the AWD system can function as designed. I know this from my time spent with the Mitsubishi Evolution X. Which had a fantastic AWD system.
On these trucks, being 4WD. they only put power to the rear tires, unless you switch to 4WD. So in this case, i would say it is perfectly fine to only put on two new tires.
If you do only put two tires on. i would put the new ones on the back, as you will wear them faster than the front. Also, i would get an alignment done after, as is sounds like you have some uneven wear. Which could be due to camber, toe, or caster being slightly out.