03 5at sedan, 150k miles

I'm having this weird problem that's been going on for a while and it's starting to drive me nuts. I'm assuming it's the lock up clutch on the converter.
Whenever I'm up to cruising speed (everything warmed up) in 5th gear, the entire car has a slight shudder whenever I completely let off the accelerator, and then press back on it. I'm not sure how to describe it other than this slight, but noticeable shudder that's very quick, as if the car hit a slight ridge in the road. It does this both when I completely let off the accelerator, and when I press back on it.
If anyone could shed some light on what might be going on that would be great.
Austin