Can I shift from 2HI to 4HI while moving?

Quoted just to re-emphasize. Every car on the road has 4-wheel-stop. 4 wheel drive helps you get moving, and gives you a bit more control while moving...you still need to treat ice as though you have no brakes and follow at greatly increased distance, lower speed, engine brake, and know skid control maneuvers.

Indeed...the rolling resistance of the front wheels and light ass are what make 2wd pickups so prone to going ass-first on slippery surfaces....even worse is that most people start braking when the ass gets loose....

 

Yep! Kids used to get freaked out in my truck when I would unexpectedly get sideways so I would reach down, pull it into 4Hi, and then just stab it and rally car it back straight.