The GMC Sierra (and its Chevrolet Silverado twin) will be replaced this fall by a truck based on the new Chevrolet Tahoe. The 2004 Z71 Silverado that we tested had a stiff, choppy ride. Other trim lines ride slightly better. Braking is adequate, and fit and finish is unimpressive. The powertrain isn't as smooth as competing models from Toyota or Nissan. The Sierra's assets are a selectable, full-time, four-wheel-drive system and a generous load capacity. The Denali is an expensive AWD model with a 6.0-liter V8. The extended-cab hybrid version has a claimed 12 percent boost to fuel economy.