you can do that just make sure your front and rear tires are aired to equal heights so it dont bind
but then you aint gonna bind if your on snow 
yeah i never do shift at speeds of more that 25 anyway but i know that the plate says i can
the plate knows all haha
45 is the optimum shift point, and that's from 2wd high to 4wd high only of course. There should be a warning in your OM not to shift from High to Low while moving period, that could get real ugly.
As to the intended topic, I drive in 4wd at high speed all the time, never have had an issue in doing this, 4wd was not made with offroad in mind but driving in adverse conditions onroad too. Not that I've driven at 70, but 60 sure I've done that.
yeah im sure everyone knows 4L is for stopped. i mean that would be insanely stupid to try to go 45mph and shift to 4L
