I would go with heavier duty spring packs, rather than the Timbrens. Check out
http://www.sdtrucksprings.com for heavier duty leaf packs for your truck.
As an alternative to heavier leaf packs, or in addition to them, if your F250 was not ordered with the camper package, which came with the upper overload springs and bump stops, you can add them to your truck. This will also help with rear end squat when loaded, but doesn't affect the ride when unloaded, as heavier main leaf packs do.
If the springs are dead and need re-arcing, they will be sagging all time, loaded or unloaded. If the springs are not sagging when unloaded, they shouldn't need a re-arc.
When springs ride at normal height when unloaded, but sag too much when loaded, it means the springs are simply too light/soft for the weight of the load. They need to be heavier.