GM's full-sized Sierra and similar Chevrolet Silverado improved markedly last year. Compared with the last generation, engines are smaller but more powerful and economical. The ride is more comfortable, and the design of the interior is more refined. In extended-cab models, the rear seating has improved dramatically. A fourth door is available for 2000. Four-wheel-drive models offer an optional Autotrac system, which can remain engaged in standby mode and divert traction to the front wheels as needed--a welcome first in pickups.