Agree with everyone else here - check balance first and make sure you go to a reputable shop.

I had a very similar problem. As soon as I put on my winters I noticed a vibration at 55-65 that went away above and below these speeds. All was well with my 19" summer set up the morining of the switch so I knew it was tire/wheel related. The winters were supposedly balanced the day of mounting - I was a bit dubious about this so I went to another tire shop and had them checked and all four wheels were out of balance. No problems since then.....I did get the original tire shop to refund my mounting and balancing money and reimburse me for the work at the second shop.