I saw this video of what I think is a South African 70 (?) series that has a cool system.
Check out around 3:18. The way he explains it, you lock the hubs manually with a wrench, which makes them selectable from the cab, and what that implies is that it is 2wd when not locked at the front hub.
Now, not knowing anything about the 79 hubs, I'm guessing it is a lot more like the drive arrangement on an 80 series, with a solid front axle and not IFS, where the stub axle engages a drive plate on the outside of the hub, so by replacing the drive plate with an appropriate selectable hub, you can have the option of 2 or 4wd.
I'm not entirely sure what this would gain most people, probably not fuel economy, since you're still driving the front driveshaft, diff and axles all the time.
And of course, this won't help your 200/201 since we use a completely different front IFS, drive axle/system where locking hubs don't make any mechanical sense (the drive axle is engaged with the drive hub - that the brake/wheel attaches to - using splines in the hub: no removable drive flange on the outside).
But it is cool.
I suppose you could remove the front drive shaft...? That'd put all power to the rear wheels. But not selectable...