Rough ride between 40-55 mph

Hey all,
03 CL with 177,000 miles. Since doing an oil change & tire rotation last week I'm getting a not-so-smooth ride when driving between 40-55 mph. Not quite a bounce but not quite a vibration, I feel it a little in the steering wheel but mostly under the seat, as if the whole car is affected.

Any slower or any faster than that speed range and the car feels fine. Any ideas on what to look for?

Thanks,