Yup, I have found that greatly reduced tire pressure solves the washboard road but not for long runs, it will waste the tires. With clapped out original springs it can be decent. I have custom alcans built for a soa with a winch & ls in front and a 400# load in back was how they were built. FWIW...
IMO an fj40 will never ride nice or even decent, now with plenty of adjustment you can get a comfortable ride. Power steering, longer leaf springs with custom mounts, longer wb, custom leaf springs, Expensive shocks & shock mounts or just link it up, use coilovers and drive like an rc car.