The older SRWs vs the newer ones are rated a lot different even though they have the same limation ... tire capacity. Not all of the trucks come with the same rated E tires although they are Es. E rated tires can go everywhere from 3000 - 3750 in OEM form. I hear at least Rams come with Es now. When I bought my previous truck in '06 all I could find was D rated tires on Ram SRWs.
For sure, Fords used to have the highest rated tires followed by GM and then finally Dodge/Ram. In spite of what the tags say, your first look should be at the tires. The weight difference for frame/body/axle on the rear will be similar. You can see the capacity could be as much as 1500 lbs if the tires are that far off. The other difference in capacity is diesel vs gasoline.
On the other hand, if you want a maximum hauling SRW, do a SRW conversion on a 450/550 and add a bed. For a normal SRW, there's always the option of replacing the wheels with 19.5s or higher rated tires/wheels.