Yes, when you shift into 4WD and the hubs lock, etc., you are then constantly in 4WD. It's a "part-time" system in the sense that it's only 4WD when you move the lever to 4WD.

Both diffs get power all the time in 4WD and all 4 wheels get power most of the time. The exception is if you lose traction at a wheel in 4WD due to the conditions you mey be driving in. If that happens, all the power to that end of the truck goes to the uselessly spinning wheel because of the open diffs and you will effectively lose all traction at that end of the truck. But the other end may still have traction.

Hope this helps.