Towing with a 3.73 F250 6.2L, will a 4.30 axle help ??

I would assume the 4.30 will help you launch. But once cruising it may not be as much of a help unless you feel you are stuck between two gears. For the hills you are just going to have to reduce your expectations. 50/55 might be the new normal rather than 60/65 when climbing. If you plan to keep the truck the $2,500 is well worth it. If you do much 4wd consider a front Truetrac differential for an extra $500.