Don't assume its the sensor, in addition to those two sensors, the other possible cause of that code is a faulty cam phaser. (I'm not saying it isn't the sensor, just that it could be something different). The phaser valve can clog and result in the cam not phasing to the desired alignment, which once the variance exceeds a preset maximum can report that code.
How many miles are on the truck? With mileage comes the possibility that the screens on the phaser could be clogged, somebody a while back actually cleaned their screens with solvent, reinstalled the actuator and it fixed the problem. With a cracked exhaust manifold, you may also want to check the wiring for both those sensors and the cam phaser/actuator. It is possible that the wiring may be damaged as a result of the hot exhaust gases hitting them....you didn't mention where the crack in the manifold was.
One other thought, did the rich condition appear AFTER the exhaust manifold cracked? If so, that rich condition may be a result of the crack not allowing the O2 sensor to see a true picture of what's happening inside during combustion. - They can suck air in, as well as expel exhaust gas out.
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