On the Defender, the functionality is there in the ECU, but none of the wiring, so you have to wire in the switches yourself. May be best to ask them what the difference is, it may be in the pinout of the connector.
Don't do it! Get a D3. All my colleagues who used to swear by (and at) their 90's have recently switched to D3's. They're far superior in their handling and manoeuvre ability I'm told, and can handle the trailers much better. D4's are even more so with the newer software and gizmo's and the...
This is a good case for setting your vehicle up with front & rear facing cameras, with SD card recording on an endless loop. Very, very cheap now (1CH DVR is about £30, Camera will be similar), plus you can use the rear one as a reversing camera if you replaced the rearview with a screen/mirror.
Blimey, I've sold loads of them this week - the listing ended as it ran out of stock!
I've re-listed the Gasket kit here: TD5 Fuel Pressure Regulator Repair Kit with Gaskets | eBay UK
But it'll take a few days to be dispatched as I'll need to do another stock order with LR in the morning.
I've sold an awful lot of the repair kits on eBay, and am now doing them with the gaskets and all :) They seem to fix the problem for most people, and are cheaper to buy & do than the regulator on it's own, or the whole housing.
The later vehicles did not have the linkage fitted out of the factory to my knowledge, merely that it was available as an option (and the internals were back in the transfer box)
Difflock + TC is supposed to be fairly unstoppable :)