Mechanics say it could be either the u-joint,
Once you have heard a u joint clicking, they have a sound like nothing else. When you take your foot off the brake and start to accelerate you are shifting the truck weight from the front to the rear, so it might be a front end part getting some slop. I'm thinking maybe a control arm?? Can't think; too much sun today(shhhh don't scare it away). lol