Not to many choices here.

You had the 7.5 or 8.8 to choose from.

1990-up 4.0 Rangers always have a 28 spline 8.8 rear.
1998-up Supercab 4x4 Rangers are almost all 8.8s as well. Regardless of the engine.
1999 up FX4 Rangers also have the 8.8 axle, but they are the 31 spline versions. They also have 4.10 or 4.56 ratio gears in them.

If you don't fit the above list, you have a 7.5 rear end.

All Ford Explorers are 8.8 rear ends!