A dual exhaust, or a better single exhaust, would likely free up breathing compared to the stock muffler. I've no doubt that Toyota being Toyota, the T100 stock muffler is just as restrictive as any of their other stock mufflers.
As a reference, I tow 2,000-3,000 lbs through PA with my truck on occassion. While my truck can most certainly maintain interstate speeds when towing it frequently downshifts and I do find myself floored, or nearly floor, wound up fairly well, going up the PA mountains. The truck is not straining hard, but it's pretty clear that it's at the upper limit of comfortable towing.
If you look in the archives, you'll see a number of threads discussing the sensitivity of the transmission to load and downshifting. It would seem that with the truck empty it is barely able to hold 4th gear. The slightest increase in drag or load and it downshifts.
CJM100, you might be interested to note that the factory owners manual recommends disengaging OD only to increase engine braking. Otherwise, Toyota does *not* recommend disengaging it. Myself, I've found the trucks hydraulic brakes to be more than adequate for stopping a trailer.
Now there are archived threads where some folks with temperature gauges installed in their transmission lines have found high temperatures at high loads. Not disputing them. I have not had that problem when towing myself. The owners manual also specifically recommends not manually downshift as that causes overheating. The owners manual specifically advises using D and letting the transmission select the gear for the load and the temperature.
