my 83' has a little plate that says that the 4 wheel drive is ment to be shifted on the fly at speeds of 50 so no worrys
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.
