I recently bought a 2001 Z71. The manual says when shifting from 2hi to
4Lo it's best to shift going 1 to 3 mph. Why? And you can shift from 2hi to 4hi at any speed. It has the electronic transfer case.I also own a 1989 Mazda B200i 4x4 with a manual transfer case. I would never change over to 4/wheeldrive unless I was completely stopped.