One thing comes to mind immediately.
If you already know for sure the transmission is getting "hot", then installing a
Tru-Cool - Max LPD47391 47391 Low Pressure Drop Transmission Oil Cooler would be a good idea.
Installing this cooler would be a good idea, regardless.
However, if your transmission is getting hot under normal driving, something else may be wrong.
If the transmission has not had regular service during it's lifetime, the fluid is most likely worn out and the filter may be past useful.
At this point, I would suggest dropping the pan, changing the filter and topping off with fresh fluid.
Don't waste any money doing a transmission "flush".
It's way too late in the game for a flush and fill.
Odds are your transmission is getting close to needing a rebuild.
So, drop the pan, change the filter, top off the fluid with fresh and install the cooler mentioned above.
Then drive the truck until you are ready to take your trip.
If the transmission runs much cooler, then that is great.
Still, start preparing for a transmission rebuild at some point.
I'm really surprised that 2004 hasn't already been rebuilt by now.
I rebuilt my 2004 back in 2012.