One problem related to underbody shield has been reported for the 2000 GMC Sierra. The most recently reported issues are listed below.
Driving truck in town applied brakes at a stoplight brake pedal went straight to the floor. No brakes at all had to go thru the red light and around 3 cars in front of me, rubbed the curb to get the truck to stop. Rust and corrosion on the steel rear brake lines caused line failure.