steering shakes - balance or align?

I replaced the wheels & tires on my '03 Toyota Matrix with another set of
alloy wheels & tires. I bought the wheels used from someone, and bought the
tires new and had them put on the wheels at a garage. The steering wheel
sometimes shakes/vibrates when I drive over 65 mph. It shakes enough that
it's annoying to my hands, but you don't feel/hear it in the rest of the
car. Sometimes it shakes and then goes away after a little while (esp if I
slow down below 65 mph). What causes this? Did the garage not balance the
tires right? Or is something else a problem?

Thx.

-John
(remove the pachyderm)