She really bogs down at about 1600-1900 in every gear with the A/C on. If it off, she runs great! Any ideas?

 

I haven't had this problem with my F150 but I did on my Focus and it was the IAC. It controls the idle when the AC is on and in other situations.

 

The IAC only has an effect at idle speed. Off idle, it is not doing anything other than just sitting there.

Have the AC system properly checked out. Sometimes, if the charge is at specific low levels, it can cause a greater load on the engine.

Also, you might be developing one or more weak cylinders (usually due to degraded coils). These usually manifest as a low-grade misfire when the engine is being lugged at lower RPMs. Just after it shifts into overdrive with light throttle, usually around 45 mph, is the most common time it shows up.