There is no way to pin point the problem here. Trial and error is the only thing that will help you here. One thing you did touch on but seemed to undermine is the wheels. and tires. a warped tire can be the culprit as well as a bent rim. Before my sc300 i had a 95 Maxima that had vibration in the steering wheel when i was at highway speeds and eventually experiences a blow out. Just be care full driving your car when its like that cause it may be sign for bigger problems to come.

 

To add a little more info. I had vibrations above 60 mph on a 94 LS400.
It turns out that the wheels had to be retrued. They are factory stock wheels --- not chromes or anything fancy.
Retruing the wheels completely solved the problem.