I hate to say it but it sounds like your trans is toast.
![Sad :(](https://board.marlincrawler.com/Smileys/marlin/sad.gif)
If you get another auto or yours rebuilt, yes, get a cooler! Since you hit the mud, consider remote mounting it behind the cab with it's own fan. I run a remote cooler and filter. My gauge still hits 300* on long mountain climbs so I've been eyeballing an old A/C condenser on my back porch.
![Smile :)](https://board.marlincrawler.com/Smileys/marlin/smiley.gif)