I did some reading last night. It's not your 20" tires. I have the same ones.
The payload issue is mostly due to the fact you have the 2.7. Ford used lighter axles on all 2.7's and a few of the 5.0s it seems. Your truck would have the 8.8" rear end. All the ones with the 3.5 and most of the 5.0s have the 9.75 rated at 3800.
There is a cat scale at the east husky stop, or you can try the government on on highway 11 north of the city. The only way to know if you're going to be over or not is convince the dealer to let you tow it out there and weigh everything. (taking into account this empty trailer will be ligher than loaded to camp)
