Transmission Issues. What do I do? How bad is it?

I'm sorry you're having the problem with your automatic transmission, and I know the direction to go in seems a little difficult to ascertain. However, diagnostic testing is really the only way forward to determine what has failed and what needs repaired. Yes, the fluid could use changing perhaps, but it may be too little, too late where that is concerned. If you are able to manually select gears, you could have some kind of issue within the valve body or shift solenoids. You could have an electronic or a hydraulic problem. Can't really tell until having the diagnostic testing performed. In truth, because you have been using your truck to it's full capability, it's likely that the transmission was running at a very high temperature, and that internal damage related to this could have occurred. Again, too hard to tell, and a diagnostic test is required to find out what is going on.