The transmission is the weak link for towing. Heavier vehicles get higher tow ratings partly from their greater mass, which more safely controls a given load. But the transmission sees the most stress, and keeping the ATF cool, not over heated beyond a good range, that's a big key.
So as you tow more weight, I suggest having more cooler capacity. I'd add another cooler for towing a long distance with a medium to large load. Explorer owners have reported excessive ATF temps, with the 302 models, on long trips(say 3+ hours). That tells us that the factory trans cooler isn't really enough big serious towing. Before I had read that, in 2005 I went to NC and towed home my 99 Explorer, with my Mercury. I didn't have a way to measure trans temps, but I bet I was fortunate. I towed with the trans in "D" the whole trip, and changed the fluid a short while after that.
Drive it easy, not hard on the gas taking off if possible, check the tire pressures, raise the rears a little(if like me you keep the rears lower than the front normally(37/33)).