I've got a Powerstroke (auto trans) with 95k miles on it. It has never been in the shop for anything but oil changes and minor tweaks. I tow a 3500 lb. jet boat and carry an 11 1/2 foor camper. Usually at the same time, so you can't say the tranny doesn't get a workout. Also, you can't say the tranny isn't built to take it - the powerstroke tranny is made by CAT. I have a lot of friends and aquaintances with powerstrokes and this is only the second problem I've ever heard of.
Sorry to hear of your bad luck.