You have to buy the entire "hub" assembly. It's gonna be around $130 at the minimum. Over $200 at maximum. If you can afford it, go for the Timken brand. It's simple to change. Once you have the wheel off, remove the two bolts holding the brake caliper bracket to the knuckle. You may have to heat those two bolts or the area surrounding them on the knuckle itself would be better. Ford used some serious loctite on them! A long breaker bar and some heat helps loosen them. Then tie the caliper up to the frame with some wire. Now remove the brake rotor. You'll see a large nut (32mm) that has to be removed on the end of your axle. Then you'll see three bolts holding the actual bearing/hub assembly onto the knuckle. Take those bolts out and you can tap the hub assembly loose. It may be a little stubborn so be patient.
When re-installing the brake caliper bolts, be sure to use Loctite on the bolts and torque them to spec. The proper torque spec according to my Haynes manual is 85ft lbs.
The large nut on the end of your drive axle (hub nut) is listed as 162 ft lbs in my Haynes manual.. You should probably use some Loctite on this nut as well. I did.
The 3 bolts that hold the hub onto the knuckle are torqued to 85ft lbs, according to Haynes.
You may wish to double check the torque specs before performing this work..
EDIT: I forgot to add that you'll have to unclip the ABS wire from the frame and unplug the connector. The new hub will come with a new ABS sensor, cable, and connector that simply plugs in. Easy peasy.. There is also a dust cover/shield that's held on by 3 small bolts..
Also, here's a good link with pictures: http://www.explorerforum.com/forums/....php?p=1499806
I hope this helps..