Our boat and trailer together weigh about 3500lbs. I pull it around many times during the summer with my 4.0/auto and have no problems. It's not a rocket sled when loaded up, but it does OK for what it is. I have found it to be better just to run in 4th gear (with the OD aka 5th gear locked out). You really can't expect a small 6-cylinder truck to be a scorcher with a big load on it. If you don't expect blistering perfomance and just take it easy and smooth it has no problem. I can easily keep 60-65 mph on the highway and hills aren't that bad as long as you just use the momentum to your advantage.
'03 XLT/FX4 L1 4x4 4.0 Arizona Beige Automatic (my weekend ride). '16 Ford Escape Titanium ecoboost (wifes ride) '16 Ford Escape S (my work ride)