i posted a thread not to long ago about my same problem...but i found out it was a busted belt on my driver side tire!!!! yikes a good waste of 250 bucks

 

Maybe the rims should be rebalanced my friends car not truck was doing this last winter so he thought the same thing the snow because got better after he would clean them out , a week later he got new snow tires and they had a problem with one rim that was damaged a bit and bent , the shope said that was definetley the problem . He ended up buying 4 winter rims and the problem was gone. Later found out hie girlfriend ran into a curb and didnt tell him . So its possible one of your rims could be damaged , hopefully NOT. Also make sure all your weights are on .

 

I've had this also, but only after going through REALLY deep snow, just cleaning them out or letting them thaw corrects the vibration (provided your tires/wheels were balanced to begin with).