Only the earlier boxes (99-01ish) had the internal parts for the diff lock but lacked the lever.
These are easy to convert, you can use stock parts or simply add an external lever.
02's lacked all the internals too and require a replacement transfer box to add a center diff lock.
Theory was that the traction control system would mimic the effects of the diff lock.
Diff lock was reinstated sometime in 03/04 (I think with the facelift).
Don't have access to parts cat from work so cant tell you exact chassis number changes but the information is out there.
Rgds
Simon