I have said it before, and so have others on the board, and I will say it again. Undercoating is a GREAT idea, if you do it right, and one of the most unsafe things you can do to your jeep if you half azz it.
Either take the tube off, and have the entire frame and bottom of the tub undercoated, or don't bother. Otherwise, the frame will still rott at the top, and than move down and rot from the inside out. With the undercoat it will still rott just as fast if the top is not done, but you will never see it and never know how bad it is, until one day you get hit, or the frame just gives (obviously a few years down the road..)