With the factory system only the axles and pinion gears spin when not engaged.
The ring gear and drive shaft don't spin until you engage 4x4.
It saves you some MPG this way and you don't have to manually engage the hubs.
Most shift on the fly systems are similar this way.

 

with the stock 2 piece axle, the left wheel/axle turns, and that rotation ends somewhere in the pumpkin, at the spiders/right side stub axle, i'm not sure exactly where... the 2 piece right axle ends inside the cad, so the right wheel/hub/half axle turns independent of the left.

i'm just saying that if you replace the 2 piece axle with the 1 piece axle, you'll connect up more pieces, and increase the amount of iron thats turning, which will decrease mpg a tad, and you'd have to get out of the truck to engage the manual hubs. i don't think that would be a big advantage over stock unless you were breaking front axles in hard core offroading.

for most daily drivers, i'd either fix the cad or replace with posilok. what i like most about the posilok is being able to creep along in 2wd low range w/o fighting the harsh steering that you get in 4wd.

 

That does sound like a neat feature....

 

I just installed a posilok on my 99 3500 and it works great and easy to install.