To answer the op's question I hauled 4,000lbs routinely in my 1991 F250 in the form of a very large slide in camper. I upgraded the rear tires when LT 265's came out. I had some stability issues with that high of CG on a single tire truck but other than that my only concern was that the rear drum brakes seemed to be inadequate. Since that was in the days before tow/haul mode I trained myself to hit the overdrive lockout most everytime I needed brakes. I drove all over Colorado with that rig with no trouble but I did drive slower than most everyone else. With the camper in the bed the last overload was not quite engaged. Would I recommend it to someone else? No. Do I have any regrets? No.
I went to a quarry one time that sold gravel by the pickup load and had them load until the snubbers were about an inch off of the axle. I went across the scales on the way out and found out that I had about 6,000lbs in the bed. I never did that again. I felt that the tire upgrade did upgrade the truck since as far as I could find out the only difference between my truck and the F350 dually that year was two tires and wider rear fenders. I did order the truck with the extra overloads that came on the F350.
