Other than the air locker is there any other locking differentials availible for a 2002 xterra? I am desperately looking and have been researching it for quite sometime.

If you've been researching for "quite sometime" you should have come to the conclusion that there are no options, which would be essentially correct.
The only "option" is to swap in the 31-spline axles and an open carrier from a 1999 or early 2000 V6 (non-crew-cab) V6 Frontier truck which will accept a Powertrax LockRight (or Detroit, no longer available) locker, but I really don't see the point of going to that much hassle.
http://home.sprintmail.com/~onlyone/locker.html