i haven't heard it in an xterra, just in the showroom, but one of the easiest "drop-in" solutions is the JL Audio stealth box:
http://mobile.jlaudio.com/products_stealthbox_make.php?make_id=73&model_id=536

Infinity makes a Basslink that I hear is pretty good. And it's fairly easy to mount out of the way. I haven't heard this sub myself.

Otherwise start learning how to fiberglass your own enclosure as many of us here have done in our x's. The most popular location is in the passenger side wall of the cargo area. it will most likely require some plastic and metal cutting, some wood work, some fiberglass work and then some finish work. I started mine back in June and I'm still working on it. I just don't have the time to finish it. It sounds good, but i have to do all the finish work to make it look pretty.