Upstream O2 sensors measure A/F ratio, to adjust fuel trims. Downstream O2 sensors are used to compare O2 levels after the converter to those before it. Their only job is too measure catalytic converter efficiency. So the front sensor can be ruled out.

If the rear sensor is good, and front sensor (fuel trims( don't indicate a rich mixture, then it's either the rear O2 sensor, converter going bad, or air leaking into the exhaust after the forward cat. Sometimes flooring the engine at high way speeds to try to burn off any deposits on the converter can help.

There's also "converter cleaner" you can buy but I'm skeptical about something like that.