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

You will increase your engine RPM by 12.6%.
That means if you were turning 2200 before you will now have to turn 2477RPM to achieve the same road speed.
You should certainly be able to feel the difference but the question still remains if it's what you're looking for. I predict it'll be pretty close. Should reduce the amount of downshifts required for those smaller grades and that can be nothing but better for your tranny.