Noticed a weird shimmy starting this winter when we had the 16" winter tires on- a pronounced shimmy or vibration in the steering wheel only when slowing down at around 45 mph. It doesn't do it when accelerating through that speed. Figured it was an out of balance wheel but it still does it after remounting the summer 17" OEM wheels. We had a rough winter, but no accidents or "incidents."
Any ideas?

('03 MCS with 33K miles)

Did replace the pads front and rear last fall but didn't replace the rotors...

 

Jack the car up and try to wiggle the front wheels with your hands. If it moves much at all, then the bearings might be going bad. But if you notice that the control arm is moving too much when you do this, the main bushing might be torn. Look at the axel boots too.