I can't believe no one answered this guy's questions. I just saw this thread:

1. The Fixed Cam Assembly is not sold seperate from the auto hub. The hub P/N is 40260-9Z500. Alternative is to post up on the For Sale/Wanted for a pair of hubs, lots of people have ditched the autos and gone to manual hubs.

2. The Fixed Cam Assembly is the part resposible for the engagement and disengagement of the dog-clutch inside the hub body. When torque from the CV shaft is applied, the cam pushes the dog clutch together to engage the wheel to the CV shaft. Do not use 4wd without this part.

3. You do not need to remove the wheel/tire to work on the hubs.

And your last question: Depends. The dealer is not cheap, try and get one used from this board or www.car-part.com