Hi, I'm not 100% sure but i've always shifted between 2H and 4H while moving, and always have to stop the car going between 4H and 4L. And never had a problem 92 Nissan Ti Terrano with Auto Hubs. But iv'e been in a few 4wds that say you can shift between 2H and 4H while going 40km/h or less and steering straight.
You can as long as you are going in a straight line and the hubs are locked in, bit hard to know whether auto hubs are locked or not though. Best bet is to fit manual hub locks then you won't have to deal with stupid auto hubs disengaging when they shouldn't.
Probably not as it is hard to be sure whether or not the hubs are locked. I think R50's have a drive flange instead of a locking hub so is just permanently engaged.