Shortride, I was with the "Herd" 1968 to 1970. 4th Batt. Co E (Recon) I was the RTO for a small team.
Raider1kilo was my call sign, kilo meaning I carried the radio for Raider1

I wouldn't recommend a 3.31:1 ration for towing 6,000 to 7,000 lbs. It would actually hurt the fuel mileage and put an unnecessary load on the drive-train. The 3.31:1 ratio would be ideal for hauling or towing a few hundred pounds but it mainly designed for fuel mileage. The best combination of fuel mileage and towing would be done with a 3.55:1 ratio.
As far as the electronic locking differential, I don’t know enough about it to give you any advice. You would need to talk with a Ford tech or post a question on the forum about it. Sorry I couldn’t be more help.

I have freind that is a mechanic and is experinced in changing them, just didnt know you could buy the 3.55 or 3.31 and ring and pinion from ford or you had to buy the whole rearend. Since I have 3.73 the electronic locking differntial, can I still buy a 3.31 ring and pinion and install it with the locking differntial. I dont do heavy hauling but 2 or 3 times a year, most of my driving is around town and highway driving. I live in Louisina so its pretty flat. So will 3.31 be good for towing 6,000 or 7,000 lbs couple times a year? Thanks for your help

Changing a ring and pinion set isn't something someone with no experience should try to do but yes, you can buy a new setup from your local Ford dealer. A 3.73:1 setup is not so bad that I would change it only to gain a few mpg better fuel economy. If you don't do heavy towing or hauling and want to make a noticeable improvement with fuel economy you should consider a 3.31:1 gear ratio.

If I were ordering a new Ecoboost truck I would order it with a 3.31:1 setup.

You need to keep this in mind. If you go to the expense of changing the gears it will take you a long time to recover the cost in saved fuel mileage.

can i buy 3.55 ring and pinion from ford and install it, or do i have to buy the whole rear end

Neil, I don't think anyone makes an aftermarket 3.55:1 for our trucks that I know of.

I have a question for you, I owned a 2011 f150 ecoboost shortbed xlt with 3.55 gears, i loved it so much i really wanted a longbed, so i placeded and order for one and sold my short bed, the long bed has 3.73 gears and getting crappy mileage out of it, can I buy a aftermarket 3.55 gear ring and pinion to change it out or do I have to go to dealer and order it as it will be more expensive . Thanks Neil