YoGeorge is correct in saying the towing rpm's of lets say 4000 are not harmful.
The alternative is lugging a motor and that is harmful.
Ford has built these newer generational modular motors to run at a higher rpm and last long doing it.
Seems the olden days of larger cubic inch motors that willingly pulled well at lower rpm ranges are gone.
Regular servicing of the motor and drivetrain should offer many trouble free miles even making it work as trucks are meant to.

 

Well I bought a blower today. Hopefully the truck will tow better now. If not, gears are next.

 

So what kind did you buy?

 

Picked up a second hand Roush along with a few other goodies (injectors, etc..) for $930.

 

What a steal. I can't imagine you'll be anything less than completely happy with it.