There's a chart that gets posted up now and then that's very handy. You still have to know how many teeth/splines/ribs on the "drive" gear however, in order to make the correct choice for the driven gear on the cable.
Maybe someone has a quick link to it and we can see it again.
In the meantime maybe take a peek into your transfer case and see if you can determine if you have a 6t or 7t spiral drive gear.
To that end, what year is your Bronco? Or more importantly, what year is your transfer case and what gear ratio came in the differentials originally? Someone may know what you have without having to stick your head up into the tunnel and count.
Paul