99 to 2000 was engine and body and interior, but the steering wheel and radios and stuff did not change much until 03, 03 were the biggest changes when they started using computers to control stuff, like ac and drive by wire, also new designed seats, and options changed. like navi became avaliable. :umn: the dash design also changed in 03, different type of radio, or newer designes, if you got 4wd in an 02 and back the stabilitrak button was where it is now on any 03 and up gmt 800 but if you didnt get 4wd on an 02 and back the stabilitrak was one button on where the 4wd would be. in 03 the steering wheel changed as well as the door panals and type of metrials used. :umn:
i personally think the ride is much smoother on the 00-06, but i have auto ride, that also became avaliable, it was not available on 99s. steering is lighter and easier to turn, but that may be different on an older gmt 800 as mine is only about year and 3 month old. if your gunna tow i would get the 5.3 as it has more torque and more HP. also i would look for atleast 3.73 gears and if at all possable get 4.10 like me as it makes towing much more comfortable with the low end torque, i tow 6000 pounds with moderate ease.