Not very, if you're moving at all, especially shifting in/out of 4Lo. usually, I come to a complete stop for that. When shifting in/out of 4Hi, you can be rolling in Neutral. Not sure how fast is OK, but I don't hesitate to do so up to 15-20 MPH.
PS-fill out your profile so we know more about what you have and can help better. /forums/images/graemlins/peace.gif