1996 Ford Ranger Frozen Suspension?

I live in Iowa and it has been very cold recently. I have had my truck for a number of years now and have never had this problem. It acts as if my rear suspension is frozen or locked and makes it a very bumpy and bouncy ride with every bump in the road. How can I get this fixed or is this just a factor of living in the balmy state of Iowa during the winter? Sometimes it comes out of its "frozen" state and is bearable, but this morning was terrible with how bouncy and bumpy my ride into work was.