Does the speed of the sound increase and decrease as the speed increases and decreases? If so, it is most likely tires. I would check your tire pressure to make sure that they are adequately inflated as well. Sometimes this type of sound can come from under-inflation. Under-inflation causes high levels of undue heat and sidewall stress to the tire (NOT GOOD AT ALL). This can cause belts inside the tire to seperate from the rubber basically causing the tire to become very unstable (kinda like jello) which can lead to blow out, inability to balance the tire, severe depressions and bubbles in the sidewall, tires will not wear properly etc etc.
What kind of tires are you running on the truck now? Some just make that loud sound because of the way the tread strikes the pavement. Usually when this is the case it gets louder and increases in speed the faster you go.
My advice would be check tire pressure on all tires, look for flat spots, check for even treadwear, look for excessive depressions, check for bulges of any size. If you find any of these definitely take corrective action asap. You can also give the tire a swift kick right where tread and sidewall meet to see if the tire has any belts broken in the sidewall (again it will jiggle kinda like jello if this is the case). Just make sure you have adequate pressure in the tires since under-inflation will cause a similar effect when the tire is kicked.