hi there! '09 2IS 195k miles, so this is one of two of my big problems, other will be listed on another thread to separate the two issues.
So around 50-60 mph the car will vibrate until I apply or reduce throttle pressure, then goes on driving as normal like nothing happened. Doesn't happen at any other speeds / gears (noticing it's mainly at 5th/6th).
Took it to my guy to take a look over, no codes / lights, engine mounts seemed all in order and still in great condition. No transmission issues, any ideas?
Done a number of fixes to make up for the previous owner being a butt but otherwise she runs quite nicely for a 14yr old car!

 

Did you figure this out? Mine just started to do the same thing. I have over 310k miles on mine. I'm suspecting it's the joints on the driveshaft are going bad.

 

There was a another thread just like this recently and it ended up being the transmission. To eliminate this as a cause here, we need to keep thr transmission out of torque converter lockup.

My question to each of you, does it do this when cold? i.e. take off from being parked for 4 hours and vibrate before the temperature gauge moves from resting position? Does the vibration go away if you lightly tap and ride the brake? Brake light must come on for disable lockup.

Do the basics:
Tires balanced. Move tires front to rear.
Verify rims and tires have no runout.
Some cold tires take a set and vibrate. Does it go away when warm?

 

Everything is fine as OP but the vibration only is happening when getting up to highway speed. Accelerating under load 5th gear to 6th.

 

Hmm... driveshaft carrier bearing?