I've engaged 4wd on the fly at up to about 70kph, you have to make sure your back wheels aren't "spinning" as in running at a different speed to your front, i just let off the accelrator pedal a second before i engage to let everything even up. I also make sure i'm going in a straight line.
Can you imagine putting a load on the roof but, you'd scrape your doors on every corner
1989 GQ TD42 N/A
Old enough to know better, young enough to say what the hell.