So I went online and starting reading about clean diesel. I am very reactive to diesel exhaust, so if I go this route, I need to make sure the engines in the heavier pickups meet my standards, but it's an exciting possibility that would give me some more leeway with weight. I still need to keep the whole thing under the limit that would require me to get a commercial license, but I wouldn't be weighing every ounce.
Here's what I found:
Effective Emissions Control Technology
Introduction of ultra-low sulfur diesel fuels for both on- and off-road applications has been a central part of the new clean diesel system designed to meet near zero emissions standards. With the introduction of lower sulfur diesel fuel, a number of exhaust treatment systems such as particulate filters, exhaust gas recirculation (EGR), selective catalyst reduction (SCR) and diesel oxidation catalysts (DOC) can further reduce emissions from diesel engines. The installation of various emission control technologies may also improve emissions from older diesel engines through retrofit capabilities. Read more about retrofit capabilities to reduce emissions from older vehicles and equipment.
I have not yet been able to find out exactly what the heavier trucks I'm looking at are fitted out with, but hope to know in the next few days.