As Ron said, multiple choices for your axle.
First, what year is it? And if you go by the vehicle year, are you certain that it's the original axle?
You can see some of the info you need here:
http://www.wildhorses4x4.com/category/bronco_rear_axle
If you don't know for sure, there are charts showing the flange offset so you can measure to be sure.
The next, and most common issue is the size of the bearing. There were two sizes throughout the years. Just called "small bearing" and "big bearing" and yours could have either one.
When you get it out you can measure yours, and may even find a part number stamped into the ring. That'll tell the tale on bearing size better than any guessing from outward clues on the housing and brakes.
Paul