I was not aware that they offered an AWD Suburban in 2004. My guess is you have an "Auto" setting that is AWD and is safe on hard surfaces (Autotrac). However, when you're in "4 HI", or "4 LOW", the center diff is locked. It also should be capable of "2 HI" for the street. Most of the time you can get away with the auto setting and again, it's safe to run it in "Auto" on the street. If you're having a hard time finding traction on snow or really loose surfaces, switch to "4 HI". You'll probably never need "4 LOW" unless you just want to creep along slowly over something really rough.

 

mine is AWD and works great, I don't do any off roading. but in 8 inch of snow with ice under it, and it just takes off. mine you can also put it in 4lo and turn off the stabiltrac and traction control. just remeber the t-case is a must to keep the fluid changed. the t-case is wokring all the time. my MPG seem to be the same as a non AWD.