I think that it's if, and that's a big if, the rear locker can be added it will be very expensive. From what I have read, along with the locking differential, a different harness may be needed and a software change. The locking function is varied depending on conditions and controlled by the computer so its a lot different than just install a new diff. and then hitting your favorite trail.
The LR3 is a very capable off-roader without the rear locker so I don't there is that big of a difference by having the locking rear. If somebody on here has experience with the regular and HD version I'd love to hear their impressions.