The reason they make 2wd Xterras is as much marketing as anything else. With SUV's being one of the largest vehicle market segments (despite rising fuel prices, etc.), why miss out on a potentially large audience by only making a 4x4 version?

Around here in the cotton belt, there are three types of people that use 4x4 on a regular basis: construction (pickups are the norm), hunting (ditto), and off-roaders. All of the off-road clubs in our state cater to extreme rock crawlers, most of which drive vehicles that barely resemble the vehicles they are meant to represent. 99% of those are Jeep-based.

Our local Nissan dealer only orders 4x2 versions of the Xterra and Pathfinder for new vehicle sales. They order a smattering of 4x4 Frontiers and Titans, because they know they might sell a few to hunters and construction workers.