The rattle happens on bumps but does it also occur when putting it in gear, moving from Park to Reverse or Drive? Does it ever occur when the engine is idling but the truck is stationary?
As you've seen, it can be many things. Common issues do include the mounts for the front diff, both at the crossmember at the rear end of the front diff but also at each axle. Make sure the crossmember itself is tight as there are four bolts securing it to the frame. Loose shock mounts could cause this issue, though a worn shock alone probably isn't likely to make these noises.
As Bigunit said, loose skid plates are a common source for noises like this. Some people have had the brake line that runs along the top of the firewall pop out of the clips and rattle.