I shift into 4wd all the time while going 50, and I have not had any problems yet.
I never do it going any faster than 50, because usually I limit my 4wd speed to 50 mph. This is during winter when if not in 4wd, I will be in the ditch. Why not shift while going that fast? It never resists when I do it, and never makes bad noises either.
