Where is the expansion valve for rear air in a 1999 GMC Suburban?

There's no single "expansion valve" for the rear air conditioning in a 1999 GMC Suburban (or most vehicles with rear AC). Instead, the rear AC system relies on the front AC system's components and a separate blend door/actuator to control the airflow to the rear vents. There's no separate refrigerant circuit or expansion valve for the rear.

The refrigerant flows from the compressor, through the condenser, receiver/dryer, and evaporator (all shared with the front system). Then a separate ductwork system distributes the cooled air to the rear of the vehicle. A blend door and its actuator control the amount of cool air sent to the rear. Problems with rear AC often involve issues with this blend door, the associated wiring/actuator, or blockages in the rear ductwork, rather than a failing expansion valve.