Need help with P2A00.

So this is a little long. The code is for Oxygen sensor out of range, but I think that may just be a symptom, not a cause. Here's some facts:

• 2006 Titan LE, 171k miles
• Cajun b pipes, uprev tune (installed by previous owner)
• New front wideband O2 sensors (installed by me, very recently)
• one O2 sensor routinely reads 0.3 volts higher than the other (this may be related to the problem?)
• There's an exhaust leak south of the primary O2 sensor on the passenger side still. I'm waiting on a gasket to arrive to fix this. I have fixed the other exhaust leaks.
• I have found and fixed several intake leaks that came with the truck. I'm waiting on new PCV valves to arrive in case the old ones are worn out (they probably are).

Anyway, I thought that getting rid of that last intake leak would fix my problem with this code showing up. And it largely has. My short term fuel trims are much better now. However, after about 350 miles, my long term fuel trims are back up around +8%.

That's the best quick summary I can give. Three symptoms: P2A00, +8% LTFT, one O2 sensor is reading 0.3V higher than the other.

Anyone have any idea what's going on with this truck?