It could easily be the transmission. When was the last time that you completely changed the transmission fluid and filter and serviced the complete drivetrain? In the 7.3 PSDs I've worked on, using Amsoil or Mobil 1 full-synthetic ATF (17.7 qts) has tremendously improved transmission towing performance, even in 100 degree+ weather. Be sure to also change the differential gear oil every 100,000 miles. Add 8 oz. of Friction Modifier (XL-3, Blue) first. Then, add 7.5 pts. Of 75W-140 GL-5 Synthetic.
If you still think it's your injectors, this link will tell you how to easily test and replace injectors:
https://www.askmehelpdesk.com/cars-t...ml#post1415281