Shift on the fly?

[Bluto74]

My 84 with the 727 and np229 says to stop the jeep before switching between 2 and 4 wheel drive. I know that later models have a shift on the fly switch. what makes this possible on later units? from just looking the systems seem the same with only the switch box being diff. the later years not having a "pull down latch" on the dash swith box.

[talntar]

you have a vacume disconnect on the front axle that needs to engage before you move otherwise things go boom.later models the front is always engaged so you dont need to stop and pull the knob.