Tom, I'd wonder more what your differential ratios are on the truck. Often guys put big meats on without considering the impact to overall driveline ratio. Can you tell me what rpm you're turning at a specific speed in Drive (not OD), like maybe 60mph or 50mph? Really any speed will do, just get it steady state and mark the rpm. Then drop it into OD and note the rpm change. With those we can quickly figure out the ratio and see if that's your problem.