If the vibration is most noticeable around 50-60 mph, then it is usually a tire out of balance, as previously suggested. If it vibrates all the time, I'd suspect bent wheel or bad tire.
You don't say how old the tires are and how much tread left. One of the ways that tires get out of balance however, is that the alignment is off and the tires tread gets worn away in abnormal fashion. So although the balancing the tire might fix the immediate problem, if your alignment is off the problem will come back again.
Many alignment shops will check your alignment for you, and only charge you if it actually needs alignment, so you may want to check around in your area of the country to see if that is their local practice. You need to evaluate how much tread life you have left on those tires, to help you determine your course of action. May not make sense to balance a set of tires that are almost gone, if you only do local low speed driving. Save the alignment and balance until you get a new set of tires. If the tires have a lot of tread life left, get them rebalanced and check the alignment.