The imrc is a separate set of throttle valves that are between your intake and each head. They are controlled by an electric motor which is attached to the back of your intake. At lower loads they close and partially block your intake runners which increases air speed. When you throttle down, they open all the way up and let all the air go thru.
Sounds like they may be stuck open on one bank. There are little rods that connect to them and open and close them. I'm not sure if the rods can be seen with the intake is on. My intake is currently off. If some pics would help, let me know.