What is causing my 2000 Chevy 1500 2 door 4x4 5LT motor to shimmy when you press the gas, but stops when you hit the brakes/

Could be a miss-fire in the engine or a transmission problem. I would check the transmission fluid level and take a close look at the spark plug wiring and coil connections. Also it might be helpful to start the vehicle in total darkness, raise the hood and watch for blue arcing at the plugwires. You should not see any sparks or arcing.