The side vents arent blowing any air in the back only the front passenger and driver
My car has 154645 miles.
My car's transmission is unknown to me.
Check to see if the air is blowing out of the floor vents in the rear? If they are, then you have a bad rear air diverter actuator in the back on the evaporator case.
If there are no blowers blowing, then you may have a bad rear blower assembly. It is in the rear evaporator blower housing and may also need the blower resistor.
YourMechanic can send a mechanic out to you for a diagnostic inspection of the air not coming from the vents in your Yukon, and recommend the best path to repair.