I'm not 100% sure on this but I don't think there are any available yet, I did read something a while back that the D40 suffers the same issue as the D22, the diff isn't really strong enough for a locker, not that it bothers me, I'm planning on grabbing one for my D22 when I get a chance.
I just got an e-mail from 4wd Systems, they have released a front auto diff lock for the D40 Navara and pathfinder which will be available in December. I'll be ordering mine next week when I get paid.