Hey, I've looked around and still haven't got much useful advice, so here's my situation:

When playing barre chords (especially sus4, or any that use two fingers to barre) my wrist gets tired and cramps easily, even more so on guitars with high action. I've been playing guitar for about six years and I've always had this problem, tried multiple ways to fix my technique, but not much was gained. So I was wondering, is there a problem in how i position my arm, should i just get stronger in my forearm/hand, or am I not practicing barre chords enough?
It sounds like you need to change your thumb position on the back of the neck. Moving your thumb while you play helps relieve tension, and furthers reach.