Ford E350 rear AC isn't cold

[sixto]

My brother-in-law's 2006 E350 passenger fan has cold AC from the dash vents but ambient air from the ceiling vents. The system has front and rear blowers, blower dials and evaporators but a single thermostat dial. I think either the rear expansion valve is bad (it would have a rear expansion valve if it has a rear evaporator, right?) or the evaporator is beyond clogged. There's good airflow from the ceiling vents so it has to be a bad expansion valve. Anything else it could be? The van's in the shop today.

Sixto
87 300D^2