Sounds like it very well may be the mode switch. Hard, even violent shifts both up and down occur when it goes out. Check also if your shifting display on the dash flickers oddly when you do shift. Sometimes it is more apparent when you drive the truck first thing. If you do a search on Mode Switch you will find tons of information. Here is just one link:
http://www.vehicross.info/forums/sho...ht=mode+switch
Good luck with your VX