Hello Charlie, check your rear tires, see what the rated capacity is at max pressure. You can overinflate the tires by 5+ psi, this will increase the load capacity and keep them cooler.
The big difference between a 1/2 ton and a 3/4 ton is the axle size and usually the number of axle bearings. Most 3/4 tons have two bearings at each wheel, a 1/2 ton has a single bearing, like a car axle.
I've run some very heavy, overloaded truck in the past, but that's me, I don't recommend going over 1500-2000#. You don't want the rear axle touching the stops on the frame, this is a guaranteed broken axle.
What you can do to help the load, would be to make some riser-board sides for the bed, and load the bed deepest at the front, just behind the cab, and leave the load shallow at the rear of the bed next to the tailgate. This will put a lot more of the load up front onto the front axle. This way both axles and sets of tires will share the load instead of mostly the rear axle.
Put a tarp around the inside of the front of the bed and the riser boards, this will be held in place by the weight of the coal, then drag and drape the tarp over the load to the rear, tying it down, or use several bags of bagged coal to weigh the tarp down. You DO want to cover the load to keep the police at bay.
I routinely put 1500-2300# in my dodge 4x4 2500 [3/4ton] pickup, and it handles it very well, it likes it even.
Keep the tires hard, to keep the flexing and heat down, take your time, and you should do fine. Buying Nut coal right??
Take care, Greg L