Vibration problems such as you describe can be particularly difficult to diagnose. Assuming that your suspension is tight and properly aligned as described above, the most frequent condition causing a vibration at your stated speed range is one of your prop shafts or CV joints is out of balance, and they can even come that way out of the box, so wear isn't always the issue.
These symptoms can also be caused by a bent spindle or wheel, or even something as innocous as a "flat spot" on a tire, caused by an emergency stop.
Most new car dealerships have a set of instruments that conduct a "frequency analysis" on vibration issues while the car is being driven, and point the technician toward where the problem lies.......unless it's something simple, that procedure is what I'd suggest.
doc