Ha! Never thought of using callipygian for a truck, John!
Actually, I think of those pickup trucks that have an extra wheel on the back end as the quintessential big-ass trucks. They look like one of our (rest her soul, deceased) guinea pigs, who had a huge rear end, but a normal middle and front. (You might say she had a fine badonkadonk.) We call them Pfeffer trucks in her honor.