we had an older diesel with the 3.55 rear end. it was a dually. I turned the pump up one day after milking just for fun. hauling round bales we could easily get over 40k lbs of rolling weight. after smoking a couple trannies we got an auxilary cooler. it was about a 12" by 12" with an electric fan. just put in a toggle switch to control it. ever since never had a problem. one of my buddies bought the truck and has been pulling a huge fith wheel camper trailer with it. just change the oil and filter once a year and he has been a good old truck.