I have had the same problem before and a wheel alignment fixed it. I specifically remember it only happened at higher speeds (above around 80km/h) Obviously it could be a number of other things though e.g. suspension.
wheel alignment would have nothing to do with a viabration, the wheel alignment stops the car from pulling to one side and wearing out tyres. a wheel BALANCE on the other hand fixes viabration in the steering wheel!!
i used to be a tyre fitter i can tell u that if u get ur tyres balanced and a wheel alignment it will fix it and if it doesent then its sumfin with ur suspension but thats only very rare.