4000 lbs. when loaded plus the 3,000 lbs of the truck when empty on the rear drive wheels puts a load of 3500 lbs. per tire in the rear. Stock tires will be at 3200 lbs. or less.
Toyo or Nitto have 3750 rated tires or you can go to 19.5 rims and tires and get 4400 lbs. per tire. The 19.5 option will cost an additional $2500 over getting the Nitto or Toyo tires and putting them on your existing rims. These tires do require a 7.5" or wider rim so this would need to be confirmed for your factory wheels.