Sounds like you need to look into getting a tranny cooler, especially consider it if you plan on doing more hauling in the future. Keeping it cool will certainly prolong the lifespan of it for sure, in addition to regular maintenance with transmission fluid and filter changes. While this truck is very capable of hauling, to do it regularly however it will certainly use some upgrades to make sure it stays reliable IMHO, starting with a tranny cooler.
As for the temp gauge, you need to understand that the gauge is not precise in it's measurement, as long as you're not pegging in the hot limit, you should be alright. Trust me, the dash will warn you if you get too hot via the dash readout, especially if you're running A/C, as it will shut it off if it nears overheating along with an audible "ding ding ding". As me how I know... *sigh* (stupid aftermarket water pump, that thing bled coolant out of my truck like a stuck pig when it went after almost a month of install, it's Duralast or AC Delco for me here on out) My recent experience with a spectacular water pump failure scared the crap out of me, but at least it gave me a good audible warning via the dash and chimes when it failed.
Anyway, hope this helps, good luck!