G'day Joseph Michael, ARB did make an aftermarket rear air-locker for the LR4, but had so many problems with fitting them to the last couple of years of D4. They then dropped the unit & said it was not suitable for D4. ARB's front air-locker works o.k. on D4's, but without a rear locker it's of questionable value. Another Aussie company; Harrop make a brilliant E-locker, based (I believer) on the Eaton unit,
however it will not fit the D4's diff housing. I've tried to convince them to re-engineer their unit to make it suitable for D4 (and D5) but they're not interested even though the market for such a unit is expanding due to D4's becoming a little more affordable here on the used market.
You cannot fit a LR rear diff lock to an existing vehicle as it must be factory-built-in to the terrain system electronics. From this forum I recall that a couple of guys in the U.S. have successfully fitted them, but
maybe they're engineering & electronics geniuses. Dealers can't & won't do it.
I could not imagine that new Defender would be any different. Maybe contact Harrop to see if they've looked at Defenders diff housing specs.
Here in Oz I don't expect to see many new Defenders in the Bush as they're much too expensive with the
packages. The current Toyota Landcruiser 200 series & Toyota 79's are more affordable & Toyota has
dealers even in more remote parts of our vast land that can fix them - unlike Land Rover!!
If you really want a Defender that can handle every terrain imaginable then bite the bullet & pay the price
for a package with rear diff lock. I won't be - my D4 with its terrain system, Compo wheels & KO2 tires
& good protection gear can get me in and OUT of anywhere I want to go - AND it's bigger than Defender
& has a full chassis that can take all the aftermarket gear anyone could possibly need - unlike Defender.