What can be the cause of a vibrating steering wheel when driving faster than a specific velocity (let's said 60 mph). How to discriminate one cause from on other ?
Vibration felt in the steering wheel is usually caused by the front tires out of balance. That's the first place to look, other possibilities include: broken belt in the tire, out of round tire, a bent rim, a bent or out of round hub, wheel bearings.
One of the things you can do to help narrow it down would be to rotate the tires, if the vibration in the steering wheel goes away then it's related to the tires or rims. It would be just as easy to have the tires balanced though.
I had a bad vibration when cruising on highway, I did an alignment and changed my wheels bearing. The vibration was mostly gone but I got a "death wobble", which is a loud noise coming from the front at high speed. I changed my tie rod ends and steering damper, redone an alignment and now everything is fine.
I'll do what Larry suggest first, this are the cheaper and easier things to do than you look at other possibles causes.
Check your Engine balancer shaft (crankshaft hanrmonic balancer) bushes. Balancer is specifically designed to absorb harmonic engine vibrations as engine speeds rise. If the harmonic balancer gets too old or fails and can no longer properly absorb the harmonic vibrations, the engine will shake excessively..