Rubberized undercoating in a can is a total waste of money.
It doesn't much do what they tell you it will do. It will not waterproof anything, and is a rust attractor instead of a rust preventor. It you have seams under your XJ you wish to seal, I recomend latex caulk. If you have undercarriage rust, use a rust catalyist, then paint it.
It will reduce noise, a little bit, but only because it weighs something.
On the underside, you'd be better off using roofing compound than Ruberized Undercoating, if you're going to use anything.