Why dont you post a pic of the rig now with emphasis on the current wheels n such??
That might go a long way to people helping you out with your queries.
If your going over rocks of any decent size you sure as heck want some body armor. I know you are worried about gas mileage but my attitude is I can pay for the armor now and a few bucks a month of extra gas money rather than cough up un-planned $1000's on a crippled truck that I still need for my regular on-road duties. The fellas on this board sure can help save $$ with upgrades.
I gotta say too that my BFG AT's kick ass..but each to their own. My wheelin' is pretty much AT so I guess they suite me better than some other wheelers who need bigger lugs.
I have some nice aftermarket steel wheels on my rig from Discount Tire. They look pretty slick and didnt cost me much. I dont have to worry if I smack them around either.
In my opinion a good set of tires that have been worn and balanced properly as well as some well maintained wheels pretty much rules out a need for bead-locks on an XTerra.
Skid Plates, Sliders, ARB, Winch ----> Worth their weight in Gold. Im still adding to mine as I realize how smart an option they are. The T-Case on rocks is a real worry...
Cheers and good luck!