Check your rear slack adjusters. I had the left one freeze up and it caused a pretty bad pull to the right. If front end is way out of alignment, it can cause pull (check tire wear). If the truck will roll easily this doesn't necessarily mean that a caliper isn't bound up. Had one get kinda wedged in its bracket on my old truck, yikes, talk about steering pull. Check your kingpins too, if they're fried then it'll cause the affected front wheel to move around under braking. Unlikely, but cooked wheel bearings could also do it. To quickly check the kingpins and wheel bearings, jack up the front end of the truck. While looking at the kingpins and the wheel hub, pull out on the bottom of the wheel and see if anything moves around. If ya take the wheel off, (easier to see) just yank up and down on the hub and check for movement.
Hope this helps,
Sean