FYI, I replaced wife's then RX300 O2 sensor for around $75. It was one right in your face when you open the hood, in exhaust manifold. Each RX300 we looked at then had exactly same issue - CEL pointing at that O2 sensor.
I also did replace the EM to flex pipe seal, "doughnut" on it. It started making roaring noise, just like an exhaust with a hole in it. But absolutely nothing like vibration. Was PITA to do but I did it.

 

So it’s a really rough feeling, vibration, droning that you both hear and feel upon accelerating. You can feel it through the whole front of the vehicle, and in the pedal. Initially I noticed it around 40 and above, but as it got worse it was bad at all speeds, including taking off from a complete stop.

Sorry. It awfully sounds like bad engine mounts. Yes, I know. You got them replaced. Never had O2 do anything like this. Something is way out of balance in your power train.
Was timing belt ever done, btw?

 

 

Ok, so it's gone. Say prayer, amen at the end. Happy New Year, enjoy the car.

 

 

You have to wait for CEL [pending or activated] for try doing live monitoring. With live monitoring, you must have enough know how to interpret the numbers.

Flashing CEL is something that needs immediate attention.

Going into limp mode is no fun though.

Salim

 

I had this problem a few years ago and mine ended up being the OCV valves (oil control valves) which control the VVT (timing). I agree with the previous poster who commented on this with the YouTube video. these valves have no feedback to trip an error code in the computer, but if they begin to clog or stick it will throw the timing of the engine off and cause a CEL with the O2 sensors and MAF sensor eventually as the computer is trying desperately to advance/retard timing, add/trim fuel/air mix, etc.

My car ran like crap and eventually went into limp mode. Occassionally is would "miraculously" run fine for a while when the valve un-stuck, then return to its ill behavior. I replaced all O2 sensors, MAF sensor and still had problem. Mine was also throwing codes that the spark plug coil(s) were bad. I would swap them around and get the same error after resetting so I know that was BS. In the end it was all solved with $175 and two new OCVs.