GMC truck body parts?

hello, i have a 2001 gmc sierra 2500hd crew cab. i had a skid of cement buckets slide into my cab and it dented the panel below the rear window. i have no idea what it is called. does anyone know what it is called and possibly how much it will cost to buy a new one? ( i know i will have to drill out spot welds and take the bed off, but i have all the tools to do it) thank you.