Page 500 of my 2014 manual says the same thing. However, on page 340 it says "Your vehicle's rear axle is filled with a synthetic rear axle lubricant and is considered lubricated for life. These lubricants are not to be checked or changed unless a leak is suspected, service is required, or the axle has been submerged in water."
I think I found where I messed up...unless service is required...I missed that. So, service is required on the rear diff at 150,000 miles, which I guess is Ford's definition of "life.". So, when it gets to 150,000 I'll change it. Thanks for motivating me to go back into the manual and take a look.
FWIW, I just had the front diff passenger side axle seal replaced today...pesky seep. We checked the diff fluid and it is a really dark/black and well-used color...not healthy looking at all. Will be changing that out right away. Odd, though...because I have hardly ever driven the truck in 4WD...not even 5-6 miles on snow and once up a steepish muddy track...although it had about 28,000 on it when I bought it...maybe previous owner?? Truck was immaculate when I purchased it.