I'll disagree with the need for a locker on an expedition rig...especially a Rover.
Series Rovers don't suffer from this as much, but any coil-sprung Land Rover is inescapably cursed. The true achilles heel of the modern coil-sprung Land Rover is the very weak third-member. The pins are barely there, and when they go they usually plow a hole through the diff-housing. Plan on them breaking at about 150K and you limping home trailing a path of oil (like I did one morning...).
IMO, the third-members ought to be replaced with something...ANYTHING better than what came stock. The ARB (remember...I'm biased...), seems to be a good option and even if you never engage it once is a worthwhile upgrade. Honestly, my '94 LWB has them front and rear, and I've never even bothered to hook them up to the compressor...and it's been 7 years since I installed them. Never engaged once...never regretted the upgrade.
You don't hear of the weak third-member on the Series trucks, which is what the OP is clearly working on. I just wanted to add my .02 cents to the locker/no locker discussion.
Yes there are a lot of viable replacements, I just happen to like ARB.