Everyone has already said it, but 4-hi. What year is your suburban? Some had the Auto AWD option transfer case. That said, with decent tires I've never needed 4x4 on the road.
I recommend you find a slippery field and try out your vehicle in 2wd, 4hi, and 4 low. Practice shifting and see what each mode feels like. You'll find out on your own what works for what situation, and that 4-lo on the road is not appropriate.
Better to know now rather than try to figure it out later