Would you describe the vibration as:
o Coming from the front end? -> think hub bearings, wheels/tires then suspension
o Coming from the rear? -> think wheels/tires then suspension
o Coming from the center, perhaps around the back of the driver seat? -> think drive shaft center support bearing then drive shaft U-joints
o Changes when you raise/lower tire pressure (always keep the pressure the same on both sides) to max/min? -> think tires

Read the Wheels and Tires section of the 700/900 FAQ.

Rotate the tires front to back, then side to side if needed to try to isolate a bad wheel. If the problem moves or significantly changes then it's very likely wheels, in particular an out of round tire with ply separation. If you think this is it then try swapping in the spare to help confirm/isolate this.

Check the front end. Raise the front wheels. Check for front end suspension wear by grabbing the wheels at 3:00 and 9:00 o'clock and see if there's any looseness in the vertical plane. Check for loose/worn bearings by grabbing the wheels top and bottom and see if there's any looseness in the horizontal plane. If you've got a torque wrench you can easily try re-adjusting the front wheel bearings -the Haynes and Bentley manuals have the procedure.

Because road speed and load are a key factor, I'm really smelling an out of round tire, typically from ply separation or damaged sidewalls. These wheels will dynamically balance just fine, but will deform and become out of balance such as when under load or when warm. An out of balance condition then causes speed related vibration at the harmonic sweet spots. Inspect the sidewalls for lumps (temporarily lowering the tire pressure will sometimes accentuate this). Inspect the tread grooves for cracks (temporarily raising the tire pressure will accentuate this). Ply separation can be due to a manufacturing flaw or an improper flat repair. You cannot diagnose an out of round tire by spinning it and checking for diameter variation unless you've raised and properly rested the tire overnight -even then some variation is normal.
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Dave -940's, prev 740/240/140/120 You'd think I'd have learned by now