Hopefully Waldo will chime in - he's an expert on this type of problem. It could be a tire but if you've already rotated them then that's probably not it.
Another possibility is a bad wheel bearing or brake dragging. After driving the car for a long distance check the temperature of each wheel and/or rotor. If one is significantly hotter than the others than something is dragging. Or just jack up each wheel and try spinning it.
Waldo also says that sometimes tolerances within each individual part in the suspension adds up to be out of spec overall. But I don't remember the fix for that or how to identify it.