I have the D2a lever in mine, you can lock the centre diff in high or low range, by just moving the High/low range lever too the side.
The D2a system replaces the D2 high/low lever with the high/low/difflock lever, it's a cable system and work very well.
This is it, you can see the brass lever touching the floor this is where the cable from the high/low range goes the cable in the pic is the difflock activator.
http://217.34.53.228:9876/uploads/diff_lock.jpg
Ashcrofts supply this, as well as all the internals for the transfer case that hasn't got them.
It's basicly bolt off, bolt on, the internals are 225 pounds and the lever above is 245 pounds, about $900 + postage.
Baz.