I think 93 and up are shift on teh fly, for Hi, any where between 0-40 MPH and you can engage 4wd. When I drove the jeep on the street, I perfered to do it from stop, if it would not engage, I would go to a slight roll.
I shifted my pick up into 4wd last week while going 45 mph(states it can be done till 60) and I go the most god aweful sound.