A few years back My 79' had a bump when coming to a stop and I put it off till one day when I was hauling a heavy load the Rear U-Joint let go Dropping my shaft on the road and denting the crap out of it, check your joints for wear and make sure they dont have any play, If they have grease fittings make sure they are full. I let the bump go thinking it was somthing small and it ended up costing me a new drive shaft and two new U-Joints. anyways its someplace to check.
-Jimmy