2000 Nissan Maxima vibration at 60+

2000 Nissan Maxima mileage: 75,600. I purchased the car in August 2000. The first week I had it back to the service dept for a pull to the right and had an alignment done. That solved the problem, but noticed a light vibration through the steering wheel, gas pedal, and front seats. This is most noticeable at 60 mph+ and only on a really smooth road. To make this as short as possible, the dealership denies any vibration. I had a firestone shop balance the tires, then rotate, with no difference. I took it to firestone again to rebalance and there was found to be a tire out of round. These are Toyo 215/55/16's. I sent the tire to toyo and rcvd one in exchange. Balanced and mounted, but still have the problem. A Firestone technician drove the car and noticed the vibration. Two new Bridgestone's were then installed in front, which took some of the vibration out of the front seats, but overall the vibration was still present. I had the original tires remounted and suppose the next step is to call Nissan. This vibration is a bit difficult to notice at first as it feels like it may be the surface of the road being transmitted through the car (normal road feel). On the interstates in my area, it is not noticeable at first (not the smoothest around) but on the smoother repaved roads I travel on daily, everyone who has driven the car (again, service dept being the exception) readily notices the shake/vibration. Could this be a characteristic of the toyos, as the Bridgestone's that were temporarily installed on the front seemed to help a bit? The service dept said 'everything' had been checked out, but on the other hand says there is nothing to check (no problem), as they were not able to replicate what I said was happening. This is with me in the car at the same time, telling the service mgr I can feel it while he was driving. Again, this occurs on all roads but being more easily noticed, the smoother the road is, at 60mph+.