I know the door sticker does not have legal standing in the US like I have heard it does in Canada. However, the nice little men in the cars are trained to know that pickups with big butts and medium dutys are easy targets. In GA they have a 10K threshold for commercial licence. I have been at several Farm Bureau meetings where this is explained,but I do not remember the particulars.
They also know that that 4500 and 5500 trucks are commercial vehichiles nearly every time they hit the road. My company NEVER got pulled over or checked until they started driving the 4500 and the 450. They now have been chased down and ticketed 3 times in a year. When they determine you are commercial, they can write tickets till their hand gets sore. Oh you are commercial, where is your log book, where are your DOT numbers on the side of the truck, where is your health card, where is your fire extingisher, where are your flares, ECT.
As far as the 18" tires, I do not remember. They were Continentials, I believe with a 3600LB +/- weight rating.