Do you keep O/D off when towing?
I put one of these in my truck for towing and it made a huge difference. You can have 3 different programs. I have 87 trans, 87 Towing, and a 87 performance. You can program for larger tires, shifting, damn near anything and everything.
Did you put larger tires on it? I went from 31's to 33's and I could notice a difference at about 45-50mph. With the programmer it's like a whole new truck. It comes with 3 stock tunes and you can pay for custom tunes. http://dygytalworld.ehost-services139.com/php/Ford_F-Series_Gasoline_Vehicle.html