2004 Chevy Avalanche Steering works great right, not good a

My Chevrolet Avalanche (2004) has suddenly become very difficult to turn left, but always turns to the right perfectly. The power steering pump seems to be fine, because it is fully assisted when turning right. While turning left, if I keep the speed/RPM down, it can be sort of "eased" to turn left, so I can make a make-shift left turn. I am really stumped. A source online mentioned the Power Steering Flow Control Valve, yet the dealership said it is most likely the Steering Gear Box. Does this actually sound like a symptom of the Gear Box?