When I first got my 1999 ES (after having a 1995 Camry SE) I noticed a faint wobble or hesitation at around 45mph once the transmission OD locked up. It was like I was on uneven pavement, but it was on roads I knew to be smooth. At highway speeds around 70 mph, I also noticed a very slight vibration in the wheel. I was a bit annoyed as the Camry had always been absolutely glass-smooth under the same conditions.
I first thought it was possibly an out-of-round tire. But after having a mechanic friend verify the tires were all perfect, I returned to the dealer. After driving two other ESs with roughly the same mileage as mine (~1200 mi), it became evident that the ES transmission was not the same as the Camry's - here's one case where I wish it was just a gussied-up Camry!
The upshot: the Service Manager apologized and said that it was an "operating characteristic" of the transmission. I have since gotten used to it, as there are so many other things about the car that I love!