Take the car to a car wash and blast the underside and inside the wheels clean. Wet heavy snow will pack in behind and cause all kinds of vibration issues as will mud, but to an even greater degree as mud is many times heavier. The tires being air filled will absorb some imbalance without pushing it into the car, but vibration will increase linearly with speed. Ill almost bet its snow/ice/crud on the wheels causing the majority of your issue.