They carried themselves. By this I do not simply mean physically supporting their own bodies; I mean that they have to deal with keeping themselves sane, participating in daily life as individuals as well as a group. This, in my mind, was probably the most challenging part of the war.
	They carried each other. If life gets hard for someone, you try to help them, and that is exactly what these soldiers did. When O'Brien killed the VC soldier with his grenade, and felt more than a little guilty about it, Kiowa kept trying to make him feel better. This had to be incredibly frusterating, for both O'Brien and Kiowa. Kiowa knew he wasn't getting through to O'Brien, but he was persistent. On the other hand, you have O'Brien, who hears what Kiowa has to say, and who knows that he should be soothed or relieved somehow, but he can't get rid of his feeling of guilt despite Kiowa's help.
	They carried the weight of the dead. By this I mean that they not only felt guilty and sad at the death of a fellow soldier, but most of them also felt bad when they killed an enemy VC soldier. For example, Lieutenant Jimmy Cross feels terrible about the death of Ted Lavender-- so terrible, in fact, that it inspires him to try to change the way that he led his company.
	On the other hand, we all read about how horrible Tim O'Brien felt when he killed the VC soldier with a grenade. So horrible, in fact, that he had to humanize his enemy (victim?) by making up random facts about him. This is how he dealt with his guilt, but as we've seen after the war, many Vietnam veterans can't handle the guilt and the blame and simply decide to end their lives.
	These men also carried the weight of guilt. This one burden is different from all the rest, however, in that it does not originate from the men themselves. It originates from back home, the good ol' United States. Many of these men did not know what they were fighting for, but only that the people back home (the young activists, the liberally oriented politicians, etc) loathed and despised them because they were doing something that most of them had no choice in. They felt unjustifiably guilty, because they could not justify for themselves why they were in Vietnam.
	These are some of the intangible things they carried, counterparts to the more mundance concrete articles that they packed around with them. However, though these things have no physical mass and thus no physical weight, they were constantly on the mind of the young soldiers in Vietnam, and thus provided a burden of their own.