Wheel Rim problems of the 2004 Toyota Matrix

Front wheels vibrate at highway speed, if snow slush freezes in low profile steel wheel rims. This happened twice this winter to me, I first thought a wheel weight came off the shake was so bad, the dealership checked it out an said it was ice inside the back of the wheel rim, he melted it out an it was find. It happened again latter on in the winter you only feel the shaking if you bring the car up to highway speed,I have never had this problem with wheel rims before an I live in upstate n. Y. Where we get enough snow. When the ice melts or breaks out of the back of these deep dish rims the wheels stop shaking, I consider this a dangerous problem that should be addressed be fore some one gets into an accident because of it. The vehicle is a 2004 Toyota 4wd Matrix.