Make sure that the insulation under the rubber boot isn't pushing against the shift linkage. When there is a sudden power-poweroff-power situation, if there is a slight load pressing against the linkage, it'll want to pop out of gear.
Also, you can check to see if it's engaging fully by getting underneath, taking off the 9/16ths nut, popping off the round slotted plate and shifting it in/out with a pair of small vice-grips. A small light and a mirror might help to see what's up there. Oh wait, you can do it from above if you remove the center console and center seat cover...oh wait, can you? It's been a while since I've been around a stock-ish Defender.
Anyway, if it's not engaging completely you can adjust the throw a bit. Also you can back off the diff sensor out a thread or two, there's the potential interference from that as well.
This is all based off of my LT-230 swap experience into a RRC. There are a LOT of photos in the several threads I wrote over on Pirate to see what parts are involved w/out crawling underneath.