Tail lights not working even though signal and break lights do

I have a 99 GMC Suburban. LOVE it! but my beloved truck has thrown me a puzzle! Front headlights work, signal lights, break lights, and emergency lights work. But tail lights are not working. So other cars can't see me well at night. Already tried changing bulbs and checked all fuses I could find: under the hood, under the steering wheel and on the side panel on driver's side. None of the fuses seem broken. Wires that are attached to the tail lights also seem just fine. Also, the airbag warning light is also on. Not sure if it's related or not though. Your suggestions would be well appreciated from a mom trying to learn how to care for her vehicle.