Axle can take some extra load, but what is your tires rating?
I remember having 7300lb on F250 rear taking some gravel, but I bought Discoverer's tires that had the highest ratings in the size.
I had Hancock in 19.5" size on my F450 and after over 80k they still had good tread, but hauling heavy had a blow out, so eventually had to replace the rest of them even with good tread. I remember they were close to 10 years.
6 years is not much for truck tires if they are keep in shade, but check them for cracks and if in doubt, take them to reputable shop for inspection.