Compare it to the Frontier, which is a spring-over design. Why Nissan chose spring-under for the Xterra is beyond me, I'm assuming for a softer ride.
A Frontier rear axle, and some frame mods might be an option, although an expensive one.
BTW, where did Mike get hung up on the trip ? I've run every one of the trails you took (that were listed), and never had a problem (even stock, with just 31's on).