I built a conveyor belt air dam for my Tacoma, have a metal bracket where the frame is to hold its shape and the conveyor belt hangs down. I use my truck off-road plenty and you can drag the air dam all day long, doesn't phase it a bit. I did the air dam as low as the lowest point of the underside, and then just a little lower close to the wheels.
http://ecomodder.com/forum/showthrea...tml#post454423