I have always used 3rd and 2nd downshifts when I cruise down a steep hill in every imaginable vehicle and have never had any tranny problems. Like you said it saves on the brakes.
The clunking you are hearing when you shift from park could be weak U-joints. When was the last time you greased them? Grease them up and you might hear a big difference. There are three fittings on the VX drive line so it doesn't take very long.
John