1) The brakes. This heat is normal as long as it is even between sides and you don't have a stuck caliper or two. Brakes generate a lot of heat and if you tested the temperature after braking the hubs would feel hot. If you can, take the truck out to some road where you can do say 45mph for about 5 minutes and coast to a stop with little use of the brakes. If the hubs are still hot, then it isn't the brakes.
2) The front bearings. If they have too much play, are worn, are too dry or are otherwise damaged they can generate heat. The only way that I can think of that you'd get heat in both sides from the bearings is if the bearings needed to be repacked. These bearings are VERY hard (in material) and it would be a really strange coincidence to have damaged both sides.
To repack the bearings follow Dave's how-to on our main website.