Daniel-

You should have a H233B 33-spline rear axle. At this time, nobody is making a mechanical locker for the Xterra. If you really wanted to get carried away, you could go with a replacement of your 33-spline axles with some older 31-spline Nissan axles (you are looking for ones with Drum Brakes and ABS, older Frontiers are the best bet), and then you could go with a Lock-Right or Detroit locker. Check out: 4x4parts.com mechanical lockers

Your only real option now, other than going with a LSD on the rear (and f ront !) is to put in an ARB air locker. Expensive option, but its all that's out there. Good luck!