Careful with a D2 transfer box without a tune if fuel economy is your goal. I have found the opposite with consumption going up. It seems the vehicle has to work that much harder to achieve the same speed, even if the revs are lower.
I used to be able to get 480-500miles out of my 110 tank consistently with motorway driving, now its a lot more like 390-420.
However, for me, the drivability and quieter motorway driving outweighs the fuel penalty. I would imagine a tuned engine would 'pull' the box better, but that might be open to debate. Next on my list, would have done it a long time ago if it didn't ramp up insurance premiums so much !