kick it....or poke it with a long object....it if rattles....replace

Dealer checked for back pressure.

At any rate, I ordered a new cat off Ebay for $53.95 and installed it this morning.
Also ran 1/2 bottle of Seafoam through the intake (with the old cat, not the new one). Car certainly runs (and passes) much better now, but idle is still bounces a bit. Nothing serious, and I don't think I'll have any problems driving it how it is for a long time now. That is until the next thing breaks.

In my short experience with my car, it sounds like a sensor is faulty

Likely.
vacuum leak. check lines.
Plugs/Wires. Inspect/Test.
FPR. Test.
TPS. adjust.
Timing chain. Replace at 150K.
Unlikely.