2 summers ago I carried my 830 to Alaska from Florida and back on my 2002 2500 ram. Had 265/75/16s, like you, ran the hairy edge on weight. Worried daily about a blowout. When we returned we picked up a ram srw 3500 with 285/60-20s. We now have a good safety margin on the rear tires but for cost and availability reasons I considered changing to 18" wheels. As was mentioned, used oem wheels are easy to find and 18" are common but what I discovered was offset was dramatically different and with the optional 18" wheels they would not work because, in my case, they would rub on the front sway bar. A good site for comparing tires is "rimsntires.com specspro". The site does not tell you whether the offset will work but it does give you comparable diameters widths, etc. fwiw I liked the ride of the 2500 better, it had the camper package etc.
