97 Explorer front hubs problem?

I am considering buying a used 1997 Explorer sport but it has a problem with what I think is the front hub. In 2wd it starts to make a ratching noise on and off. Very loud. In 4wd - no noise. The dealer thinks it is the automatic hub. I checked and replacements are about 295.00 each. Question 1 - does this sound like a hub problem? Qustion 2 - how hard is it to replace? I'm pretty mechanical and have a wide selection of tools and Question 3, where can I get these hubs at a better price. They are electro-vacuum type. I also noticed the check engine light on. Can a problem with the hub cause this.

Thank you for your help.