I'm not going outside to check right now (it's about -40), but there's no O2 sensor on the intake, I can't picture what you're talking about. I'll look it up later.

The intake manifold is under the plastic cover on the top of the engine. Where the manifold meets the heads (on both sides) there is a gasket that you may or may not see.
Start the truck and spray some carb cleaner along this area -just a dusting, not soaking it down. If the engine surges, then you have a leak -the engine's vacuum is drawing the carb cleaner into the cylinders through the torn gasket.

The purpose of the MAF is to determine how much air is entering the engine. If there is a leak post-MAF, then the air measured is not what is actually entering the cylinders. This will lean out your air/fuel ratio and throw off your O2 sensors. The only component that the OBD can reference is the MAF -so it throws a MAF code even though the problem might be elsewhere.