The tire usually has to be on the driver side because it's the only way the arm can swing wide enough open to clear the rear hatch so it can open too. If the tire is on the passenger side, the tire would have to swing all the way around almost 180 dregrees, which is very impractical (think about parking lots or the side of a narrow roadway). I haven't seen a mass-produced design for the Xterra that has the tire mounted more toward the passenger side. I have seen some custom homemade bumpers where the tire is more in the middle, but all of the branded ones (shrockworks, hefty, maxgear, hardcore) are shifted more toward the driver side.
I think of all the brands out there, the maxgear ones seem to be made the best. He puts a lot of thought and engineering into his designs, but they aren't cheap.