If you don't haul the plywood or drywall often (typical homeowner), it's not so bad to just leave the tailgate down. Even with the 5.5' bed, there isn't much sticking past the tailgate. If you had a long way to go, or rough roads you might want to strap it in to stop it sliding backwards, but in my expereince, that's not much of an issue.

Now if you'r a contractor who hauls full sheets on the daily, that's a different issue.