Nope, sorry, sheepish. I've not heard of a difflock on automatic 4.2's either. What you get is a differential with a viscous coupling instead. So you're less likely to get stuck than with an open differential, but at the same time, it's not as good for getting unstuck.
There are, of course, aftermarket difflocks available, and rather than getting a locking centre diff, a locking or limited slip diff on the rear would make a lot more sense.
If only there was low-range... *longing sigh*