A simple search in Xterra Part Numbers turned up the numbers for Bilstiens:
http://www.automotiveforums.com/vbulletin/t3242.html
I will say that most/majority are using Bilstien shocks, others are using Rancho and a few are using OME (Old Man Emu).
Are you going to be adding a winch/ARB, Sliderz, Skid plates, or any other things that will increase the weight of the truck you should get the +20% Bilstiens. If you are going to lift get the longer ones! SLR
http://www.spencerlowracing.com/suspension.htm is the best place to buy Bilstiens from. You tell them what you plan to do and they will order you the correct ones!
Most people crank there torsion bars when they add AALs. You'll have to do a little fender trimming too if your adding any tire 31" or bigger.
-Rick