Diff lockers make a TREMENDOUS difference for the kind of rock crawling offroading that I do. I don't know that I'd go to all the expense of putting one in a 4x2 - but some people have.
I got mine from 4x4parts.com
HERE . $720 for the locker, $225 for the air compressor, wiring harness etc.
A friend helped me put mine in so that was free...
Oh, I also spent $60 for two new carrier bearings for the ARB (rather than trying to use a puller to get the bearings off the stock carrier)