I'll take your word for it on the licking bathroom floors deal. Sounds like you really know your stuff.
Lighter? weight has nothing to do with aerodynamics, drag coefficient, downforce, lift or any of that.
Why would you say that the people posting here have never had a fast vehicle or worked with aerodynamics? Just because you don't know us, doesn't mean we don't know what we are talking about.
The most important things you can do for aerodynamics in a truck is minimize frontal area (slam it in the weeds) and give it a slight rake- lower in the front. Put a cover over the bed. Don't worry about the underside- although a lower level would help signifigantly with aerodynamics. Block off the grill to reduce high pressure under the hood area (picking up front at speed- both decreasing driveability and increading frontal area and coefficient of drag) or at least build an air damn that goes down as far as you feel comfortable. The lower the better- the rules say 3"- but that's pretty low for a diesel truck with a bunch of front suspension travel (read- lots of fixing smashed stuff- but 5 or 6 inches should be ok). Get rid of protrusions- mirrors, stacks, antennas, whatever- chunk it. Keep the tires tucked under the truck.