They could possibly be sealing too tight. When you are slamming the door, there is no place for the air to escape, and the pressure makes it hard to shut the door. If you havent already, try rolling the windows down and shutting the door. If doing this makes it easier to shut the doors, then as I said before there is no place for the air to escape. Newer trucks have vents on them to release the air. My old s-10 had a release vent on the back of the cab, just below the rear window.