P0430 Not long after changing entire Exhaust and all O2 sensors

I replaced the entire exhaust system from the exhaust manifold to the tail pipe on my '04 Tundra SR5 DC with 4.7L back in January (211k miles). I replace the 4 O2 sensors with Denso's and the exhaust was Walker. All was fine until recently when it threw the P0430 code. I cleared the code and I've had a pending code come up but not enough to set off the check engine light. I use Torque Lite to monitor engine data and when I first changed the exhaust system I would notice that both post cat O2 would show a very low and steady voltage. Since the engine threw the P0430 code, I notice the Bank 2 post cat O2 voltage fluctuates almost equally to the graph image representing the pre-cat B2 sensor or at times seems to be much higher than the opposite bank voltage.
I don't hear any exhaust leaks but I am concerned that the problem could be the due to a leak in the exhaust manifold on that side since all other components are new and not leaking. Engine runs excellent & I get 14.6 mpg on last tank with combined hwy/city driving including a small amount (10 miles) of 4 wheeling on the beach.
Can the O2 sensor be at fault or some other item? Thx.