upgrades to improve towing in a 2001 Expedition?

After spending $2500 in mods to the engine in my first tow vehicle I was able to go up the same hills 10 MPH faster and improved gas mileage by 1 MPG.

It wasn't worth the $2500.

On my 2nd tow vehicle I changed out the rear end gears for $600 and was able to go up the same hills 10 MPH faster and with the same gas mileage while towing. But when not towing the gas mileage suffered.

The gears provided improved towing performance for less money.