Cam,
Something is forcing the sensor lean. Two of them probably aren't "bad" simultaneously.
Is the KOEO bias voltage at the sensor connector at least 350mV? Did you check the vacuum fitting on the LH side of the intake, just ahead of and to the left of the throttle body? That connection has 'O' rings where the block meets the intake, and the hoses connected there could also be leaking farther downstream.