Brake lockups
#1
#2
This is a fairly common problem that has to do with the rear pads corroding a little in moist air conditions. Some have reported fixing the problem by using their parking brake religiously. I believe ford also has a set of pads they can use if you experience the problem. I believe it is in a TSB somewhere. With the truck still under warranty, you should be able to get that fix for free. Do a search and I'm sure you will find all you ever wanted to know about the problem.
Good luck!
Good luck!