To further MontyMcV's answer, the Gen 2 spins its half shafts whether it is in 2WD or 4WD, in other words, all the time. Putting manual hubs on one lets the shafts become disconnected from the hubs until you need 4WD and you lock them. It might be good for an additional MPG.

The Gen 1 is best described above.

John B.