The gas tank on a 1981 Chevrolet Silverado pickup is mounted on the outside of the frame, between the rocker panel on the cab and the frame itself, and is made of metal. Over time, these tanks can corrode from rust, or get damaged in accidents, and that means that you'll have to drop the tank and replace it. This can be done on most trucks without lifting up the truck, and the only thing that makes this difficult is the weight of the tank.
Drive the truck until the gas gauge reads as low as possible to make removing the tank easier. If the old tank has a hole in it, place a drain pan underneath the tank to catch any fuel that spills out.
Crawl underneath the truck and locate the fuel filler hose on the gas tank. Remove the hose clamp on the line using the flathead screwdriver, then pull the filler neck off using your hands. Repeat the process on the vent tube hose.
Set the wood block on top of the jack pad on the jack, then lift up the jack until it contacts the bottom of the gas tank. Unbolt the tank from the truck using the 3/8-inch ratchet and socket, then slowly lower it until you can access the top of the tank. Have your assistant hold the tank steady so it doesn't tilt one direction or another.
Disconnect the electrical connections on the top of the fuel tank using your hands. Then lower the tank the rest of the way out of the chassis.
