Check the thermastat
Though I agree with everyone on this thread, I would strongly recommend pulling the thermostat and checking it as you say the core hoses don't get very hot, but your rad. hose does. Check both top and bottom radiator hoses to make sure you have flow there. I'm not one to throw parts at a problem, but the thermostat is a relatively easy and inexpensive fix, and that sounds like what your problem is, so that's a good place to start. Not a bad idea to check your blend doors like Blue and nigel says. If I remember correctly, the doors are controled by vacuum, but I could be wrong so somebody correct me if I am. If it is vacuum, you'll have to check for vacuum leaks or even the heat/ a/c switches or the vacuum pods which open and close the doors. If all of those things check out, THEN I would say it is a heater core blockage. How does the rest of the cooling system on the truck work? Please let us know how it's going. Hope this helps.