The rear O2 sensors are only for monitoring the efficiency of the convertors.
P1131 Lack of Upstream Heated Oxygen Sensor Switch - Sensor Indicates Lean - Bank No. 1.
You could have an exhaust leak, vacuum leak or a fuel injector issue on that bank.
Since the issue showed up after the installation of the X pipe. I would go over all components that were removed or touched during the installation. It could be as simple as a loose connection or fitting or a damaged O2 sensor or wire.