very few defenders come to the US with the off road pack which has the rear locker. if you want this option you are most likely having to buy one fully equipped which is almost the same price as a range rover...like 90k! if i buy one without the off road pack can i add a factory rear locker after?
A little hard to tell where you are in the process, but why not just order a Defender the way you want it? I think you can order a nicely spec'd truck w the off road pack (rear locker incl.) for around $80K. But if you really need one that quickly, one that's from coming inventory that's not spoken for (good luck), just ask your JLR Dealer if they can add it?...(doubtful). Dealers can add certain 'factory options and accessories' as 'sticker' items before you take delivery, and that's for sure how I would want that done, if it were at all possible.
'Provenance' is important to knowledgeable buyers and key to the value of vehicles in that stratum.
Also, unless you can just write a check for it and the *warranty*! doesn't matter, I would advise against doing anything 'after market' to the drive train of the vehicle!
As far as the raptor/wrangler/bronco?- what m_lars said ! Different 'strata' entirely.
Side note: When I bought my 2016 'Landmark' in 6/16, inventory had dwindled, but I made my LR guy find me one w the HD Pack. If you're going to shell-out that kind of money for one of these things, why not have it's foundation be as capable as possible right out of the box?