You may have trouble finding any information on this, simply because the definition of PTSD is itself pretty recent. I can name any number of PTSD-type cases from the Civil War in the US alone, and I'm also sure you could find a huge number of anecdotal stories from many cultures predating that.

I suspect (though I'm not an anthro type...Marc's better qualified to speak to this possibly) that many tribal cultures had their own cultural mechanisms to deal with what we now call PTSD. Warrior societies, vision/spirit quest rituals, and even something as seemingly unrelated as the berserkers might all have been tools to deal with those who were shocked or disturbed by combat. Some tribal cultures also had social "outs" for those who weren't keen on combat (and in many cases the actual demand for warriors was low enough that it was a self-selecting process).