brakes or snow/ice buildup?

I own a 2004 Ford Freestar minivan with 39,000 miles. In December after a cold snap and heavy snow my rear brakes make a cruching sounds while
breaking and some rubbing sound in rear axle slightly picking up speed after a stop. Then we got a thaw and no more crunch or rubbing sound. We recently had a new round of cold air and heavy snow in January and the issue is back. No thaw yet, so can't tell if it will go away. Is this a rear brake issue or just buildup of ice and snow? I take it in to the automotive center this weekend to inspect the rear brakes. They do a free check first thank goodness. Anyone had this issue? And is it likely the brakes going or are Fords known for ice/snow buildup in rear brake area. Any info appreicated. By the way never had an issue like this with my Subaru, and the Subaru has 95,000 extra miles on it than the Ford van. Not sure I will go with a Ford van again.
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