Real talk: no one cares about payload. Insurance companies do, and therefore I do since I need to be insurable when driving expensive cars for business, but no one else does.
My ZR2 has a 981lb (edited from 971lbs, got myself confused) payload. There are multiple ZR2s running around with campers, 35s hanging off the back, etc. that are 25%+ over GVWR. There are multiple Tacos running around similarly overloaded- a TRD Pro has a 1,135lb payload and a 6,400lb towing capacity.
Additional real talk: towing capacity is now a function of cooling capacity. The SAE J2807 testing procedure is friggin' tough on cooling systems. The Highway Gradeability portion requires ambient temps of 100 degrees at minimum, AC at max cold with outside air selected, fan running at full. Then the truck and trailer must drive at 40mph and never drop below that speed up the 11.4mi Davis Dam Grade. There must be no component failures, no check engine or alert lights, zero part failures, zero error codes, and no loss of coolant.
There are also braking and handling tests along with acceleration tests.
http://www.trucktrend.com/how-to/tow...s-the-standard
Big engine with most power also means most heat to exchange. Part of the reason the ZR2 is rated at only 5,000lbs towing is because the front bumper got rid of the air dam and reduced the low-pressure zone that helped pull heat out of the engine bay and there wasn't room in the budget to add heat rejection.
When I was looking for a truck to buy, I looked at the Ranger (because aftermarket air lockers are available and I require the ability to be fully locked) and the ZR2. I wasn't interested in the Taco because the powertrain is a dog and the seating position is weird. The Gladiator looks huge compared to my ZR2.