We have looked at tags on the rear diff. but it dose tell what ratio is.

Put the car on a lift. Mark the driveshaft on the bottom with a chalk mark. Mark a tire on the outside top. Have someone rotate the driveshaft and count the revolutions as you count 1 revolution of the tire. If, for example, the tire rotates 1 turn to 3 1/2 turns of the driveshaft, you have a 3.5 axle ratio.

Provide the VIN to your local Ford parts department. They may be able to answer the question for you.