I have something similar on my 2017 SR5. It feels like a slight shudder in the seat from 42 to 50 mph under no or very slight acceleration. It is not always present. I too thought it was the tires and replaced them at 35,000 miles (good riddance AT20's). It still had the shudder and to me felt like the torque converter quickly locking and unlocking. I took it to the dealer and they came up with some story about how it was a "learned behavior" and they disconnected the battery to clear the memory. Needless to say, I felt it on the ride home from the dealer.

So I have 2 questions:
1. Where do the dealer service people come up with these stories? Do you suppose they go back to the break room to regale the others with the outlandish explanation the just pawned of on some unknowing customer? Maybe it is a contest.
2. Could it be suspension related? Is there some sort of low frequency vibration? Maybe an engineer can educate us on harmonics and frame modulus.