Somehow I doubt that your tranny was low enough on fluid to have caused your problem, if you as thorough as you say, you should have noticed a leak as a spot in your driveway or on your garage floor. Also, if it were a blown trans cooler line you would have smelled hot trans fluid as it thoroughly lubricated the underside of you truck, it's a distinctive smell. I suspect the final drive clutch pack just "let go". Lots-o-miles or hard driving has a way of eventually wearing things out. Too much fluid will blow a seal and you would definetly have noticed that, sometimes it just leaks out past the seal but not actually "blow" it. Whatever you find, I wish you luck, but I suspect a tranny rebuild is in your future...