Sterling 10.5"

The least expensive locker for the 10.5" Sterling is the Powertrax Lock Right. The Lock Right for my 10.25" Sterling was about $300. I had it installed at the same time as the gear change.

A gear change with a Lock Right install should cost about $1000 for your 10.5", depending on local labor rates.

The Lock Right fits in the stock open carrier, which is why it costs alot less than the comparable Detroit Locker. Both the Detroit and the Lock Right are all-gear ratcheting automatic lockers. There are no clutches to wear out and no electric or air system to install to active the locking mechanism.

I've been quite happy with the Lock Right in my F350 for the last 12 years.

Remember the OP is in relative flat lands compared to us on the West Coast. We just made a loop from the Pacific coast to MN and WI in July and my fuel mileage was better than any of my more local trips due to less elevation climbs. This was why I suggested the 4.30 over 4.56 ratio for his application.

$1,000 ballpark parts and labor. I have and highly recommend Eaton TrueTrac differential. It is the only differential that both differentiates and transfers torque at the same time. Will add about $600 to the cost.

4.30 or 4.56 is about 5%. I would probably go 4.30 but 4.56 if dedicated to towing. Fine either way, even 4.88 could be considered.

I also have an open diff wondering if it would be worth switching to a locking rear at the same time. How much does just a gear change usually cost?

I run 4.56 as well. Tremendous improvement over 3.73s

I have a 02 Excursion V10 2wd with stock size tires. I made the jump from 3.73 to 4.56 about 6 months ago. I tow a 10,000# loaded 37' travel trailer. It made a tremendous difference in towing. It still wanted to downshift out of OD more than I liked after the gears so I also added a tuner/Y-Pipe/intake combo from 5 star tuning. I tow at 70mph in OD no problem. Mileage didn't improve from what I was getting with just locking OD out. I only lost about .5 mpg running without the TT, 8 mpg towing and 11.7 mpg empty. Acceleration both with and without the TT is night and day difference to say the least.

First generation Superduty F450/550's with the V10 and 4R100 auto had 4.88 and 5.38 gears. You will be just fine with 4.30 or 4.56 as far as highway cruising rpm's go.

In my gas F350, I changed from 3.55 to 4.56. That was about 12 years ago. Still have the truck. I've been very happy with the 4.56.

If my truck had an overdrive tranny, I would have gone with 4.88 instead. My truck has a C6 3-speed, so no overdrive. 4.56 was the lowest gear I could go with and still drive it on the highway, without overdrive.

calc the highway rpms you'll see at top speed when running empty and see if you're good with cruising at that rpm and let that make your decision on final gear ratio.

I would try the 3.73 gears first and then make a decision if the 4.30 are needed.

I wouldn't hesitate to put 4.56 gears in with that engine, especially if you have the older 4 speed auto. I used to have a motorhome with the V10 with 4.63 gears and smaller diameter tires than your Ex, and while mileage was not great running 17K combined, power was decent and it never felt like it revved too high on the freeway.