My suggestion is since you can't weigh the dirt, is shovel as much as your suspension allows. Look underneath at the rubber snubbers on the frame over your rear axle and make sure it doesn't bottom out on the axle/leaf. I'd leave at least an 1" minumum if you need to travel far. Shovel as much as you can as close to the front of the cab for better weight distribution. A little trick I learned from hauling dirt and gravel over the years is...lay a large tarp inside the bed and shovel everything onto the trap....as the load lightens...you can pull the tarp back and have easier access to the material....plus it makes for an easier clean-up....