This may be a stupid question, but do you have to be stopped to change into 4H or 4L and back again? Or can you be driving? Is there a maximum speed you should go besides the 50 (for 4H) and 30 (for 4L)

You have to be stopped with the transmission in neutral. Then you have to press down on the shift lever and shift into 4L. Same goes for going out of it too. No those are the recommended max speeds.

Assuming the proper surface is present, you can go just as fast in 4H as 2H.

Hi, the topic at the top of this forum, entitled, "TRANSFER CASE SHIFTING 101", just may hold the answers to the questions you have...

If the surface is so slick that you need 4hi, you shouldn't be doing as fast as you can drive in 2hi. If the surface is good enough that you can drive fast, you shouldn't be in 4hi...

Well, that depends...BUT, it's been discussed ad nauseum in other threads...

Almost al Baha trucks are 2wd. I have yet to get above 50 in mud, even with 30' rooster tails.. in snow.. have your life insurance payable to me..
above 50 mph any "slick" spot you hit, you will be way past before you know you hit it.. else you will be sideways..
I have done the sand dunes at Pismo way faster then i should have.. in 2wd..
4wd gains you nothing at high speed, but over confidence..

...and stability.

Picture this: A couple of inches of fresh powder on the road - maybe even some cinders for extra traction, 15 degrees, good tires, straight (for literally miles at a time), no hills, and you see maybe 1 other car every 30-60 minutes.

Guess what - 55 mph in 4wd is not unsafe, and it is more stable than 2wd. In the setting I described, which I've experienced several times, there is NO downside to using 4wd except for maybe marginally poorer gas mileage.

actully there is a downside.
You still have drivetrain bind, the difference is instead of breaking stuff on your truck, you have 1 or more tires slipping just a little every time you turn the wheel.. (more in turns and twisty roads then straight as an arrow)
I have delt with this in VA in the snow. Mountain backroads are not the best time to learn that 4wd on slick pavement in a corner at 30 to 35 mph causes the rear inside tire to slide..
As you brake going into the turn, weight transfers to the front, off the rear, then turn and more shifts to the outside.. now the rear inside tire has the least weight / traction & you are having "drivetrain bind" by turning in a 4wd. Tire slips, you now have 3 tires to keep your truck on the pavement, in a turn, on a slick road. Put it in 4wd, get moving, put it in 2wd for high speed driving.
As I stated before, all you gain is over confidence. If it is "more stable", or better handling or whatever in 4wd at 50+mph, I hope you never spill coffee on yourself, or have to dodge a Deer in the road.. If it is so slick you can't comfortably do it in 2wd, you shouldn't be doing it at all. 4wd does not make you turn faster. 4wd does not make you stop faster. All it will do is let you put more power to the road, before breaking loose the rear end.

There's snow in Virginia?

I never said 4wd makes you turn faster or stop faster. I've driven in snow a lot and I'm very familiar with it. Why do you keep going on and on about mountain roads and wheel slippage around corners? Who ever said anything about going around corners? I specifically said straight roads - literally for miles at a time. With no traffic.

I think we'll have to agree to disagree on this one, because I'm quite sure of my position on this one - you apparently are as well.

"Break going into the turn"
Seriously bad idea in snow unless you simply can't aviod it, unless you want to try out your driving skills at bringing the rear end back where it belongs.

I have had 7 weeks of advanced driving, most of which on skidpads in NC..
I didn't say to slam on the brakes, you lightly put pressure on them, just enough to transfer a little weight from the rear end, to the front end to assist the front tires in changing the direction of your truck. I an talking in the 5% to 10% range.. Over do it and you can skid forward, or spin..

That is very interesting and correct in CA, but in snow the story changes and all that you learned in advanced driving goes away if you want to stay on the road.

I drove my Honda 2000 at Sears Point and took the driving classes too, but if I drove that way in snow or on ice I would kill myself and possibly someone else.