www.car-part.com
search for a carrier(see also differential) for a 2001 nissan xterra
it will then ask you what diff you are looking for. click:
Front Axle, (6 cyl), SE (4.636 ratio)
it will bring up a list of junkyards that have the front differential assembly you need. call them, and have them absolutely 100% make sure that you are getting either the 37:8 or 51:11 ring and pinion set. tell them to even make sure they take the back cover off and check for the markings on the ring gear. i've seen a lot of yards screw that up when they inventory their product. there is a also a 49:10 set giving you obviously a 4.900 rear end ratio.
now when they ship this to you, you'll look at it and say what the hell is this monstrosity. don't worry, take it apart, because all you need out of it is the ring gear and the pinion. throw the rest of it in the garbage.
this pretty much puts you exactly at the same point you would be at as if went to nissan and ordered the nismo ring and pinion. install it into your R200 pumpkin as per the FSM and you now have much better gearing for a high revving motor.
