I would personally do a half door with a very large chunk cut out.
That way you at least have a lip at the bottom, and you still technically have a door.
My Idea was to make a half door, but leave the the upper part around the window, and put on some snaps all around it and make a canvas (or somethign else) cover to go over the whole door (opending) with a plexi-glass window so that if it starts raining on the trail, or you come to a trial with alot of water, you can seal it up so you aren't filling the cab with mud.
That's just my idea though. I "want" to do this shortly.
EDIT: I was going to ask our family friend today, who is a cop here in WA, what the law was or, if he saw a truck with no doors, tube doors, or half doors, if he would pull them over, or if he thinks other cops would pull you over.
I'll probably ask him tomorrow.