If your truck didn't originally have an HD Tow package, add a transmission cooler yourself. A few bucks at the local parts store and a quart of tranny fluid is all it takes.
Might want to inspect the diff too, or at least check the fluid level. If you have a G80, remember, NO friction modifier.
Add a little Red Line Water Wetter to your radiator (drain out a litre first if you need too and throw it in the overflow tank, then add the Water Wetter to the rad, not the overflow.
And like the guys said, don't tow in OD.
I've towed some pretty big loads, pretty far. I towed around 10,000 lbs over 6,000km with our old '88 K1500 Suburban just by adding helper springs, synthetic fluids, and Water Wetter.
Towed around that much for 1,200km with our '99 K2500, no problems as well. Though that truck isn't stock. Still, it was no sweat.
Big thing is cooling and fluids.