I was thinking a lot about rear axle capacity last week, when I had to pick up 1200 lb. of seed and fertilizer. I really didn't want to hook up my tandem axle tractor, as it has solid sides and fork-lifting things in and out of it is a huge pita (has to be done with slings). My truck has a payload rating around 1480 lb., but I weigh 170 lb. and I had a nearly-full tank of gas (35 gallons). Some quick math shows I was probably right at max payload without me in the truck, but that 170 lb. discrepancy is only 2% of GVWR.
I normally never overload a vehicle, but in this case I placed the material as far forward in the bed as the crew cab configuration allows, and drove slow the five or ten miles to home. I figured at just 102% of GVWR, I'm probably within the error of measurement of any scale that'd be used to weigh the vehicle, and way within whatever safety margins were used in engineering it.