Check the Ford specs but I would bet that with the 4.30 gears your truck has a 25% greater maximum tow rating than with the 3.73 gears. I doubt when towing your truck's transmission ever gets into 6th gear.
$2500 for the new gear set is very high. The parts cost under $800 if you buy aftermarket and not from the Ford dealer and there is not that much labor to do the work. I would check with independent shops that specialize in this type of work and get more quotes. Often I have saved money by buying the parts online and having the shop do the installation. They lost the profit on the parts but still were happy to get the work.
With lower gears the engine is more often inside its powerband and their is less lugging and fewer downshifts. I have stayed with 4.10 or lower gearing on my gas powered trucks as I knew they were going to be used for towing. The lower gears also improved the offroad performance as the engine was producing more power at any given speed over the ground.
The higher gearing reflects the manufacturers' efforts to provide as high a EPA fuel mileage figure as possible and their knowing that the majority of trucks they sell are not used to tow or haul heavy loads by their owners.
