If you do change your battery, go with, IMHO, an Optima RedTop Battery Gel Cell. One of the best I believe! Get the one which I think for the V6 Xterra comes with a set of dual posts, top and side, so you can always add an auxiliary fuse block set up if you add allot of electrical accessories!
Optima RedTop Find the right battery for your truck and check out the prices here. Pretty Damn good:
BatteryJackInc