With 20" rims and tires, you should probably upgrade the rotors and calipers. Baer and Brembo make some of the best upgrade kits, but I don't know if they make anything to fit the Sport Trac. If they do they will be expensive...Probably in the $2500-$3000 range.
If they make upgrade rotor and calipers, look for the largest rotors you can fit inside the wheels. They will usually tell you what size wheel you need to fit their brake systems. At a minimum I would go with 13" rotor, and if you can find 15" rotors that would be ideal. 13" rotors usually require 17" wheels so 15" rotors should fit inside the 20" wheels you have.
You should also consider getting slotted rotors for better brake performance. If Baer or Brembo make crossed-drilled rotors then you may want to use them for the ultimate in braking, however I would not recommend using cross-drilled rotors on a street driven vehicle unless they were made by Baer or Brembo. Don't waste your time and money on cross-drilled rotors made buy unknown companies or those that are sold on ebay.
Ceramic or Carbon metalic brake pads will give you excellent braking
...Rich