[Batman]

If you drove it in the snow jack it up and make sure there isn't any frozen snow stuck on the actual piston of the strut (shock?) especially if it has a dust cover over it. Or if you can't see it grab a hair dryer and heat it for a couple minutes and see if it frees up. Had it happen before if the struts have pretty good mileage on them they extend more and leave the piston exposed. Also look for ice in your rear springs. Apart from that look for the broken parts mentioned above. There isn't alot to a rear suspension. Should be pretty easy to find if you go over it well.