The growth shown in the photo is a kind of bracket fungus, perhaps (Laetiporus sulphureus) . Information at the following website will answer most of your questions.

http://www.ipm.iastate.edu/ipm/hortnews/2007/6-27/wood_rot.html

Thanks for the information, Bob. I will consult an arborist in the area to determine the extent, severity & urgency of this condition. There is an obvious concern for the houses. If you have a recommendation, please forward the name. I hate to see it go; it provides such great shade!
As a side note, I harvested the shelves from the tree. The research I've done indicates that it should be edible. My wife is very concerned about this as it is my first time harvesting wild mushrooms.  I just want to eat the thing that is eating my tree.  Any caveats?

We do not recommend particular arborists or tree care specialists but this information may be useful:

How to hire a professional arborist to help care for your landscape trees
http://www.extension.umn.edu/garden/yard-garden/trees-shrubs/how-to-hire-a-professional-arborist/

Please do not taste the bracket fungus based on our tentative identification. If you want to pursue this, send a message and perhaps a photo to Ron Spinosa:<personal data hidden>, <personal data hidden> of the Minnesota Mycological Society.

http://minnesotamycologicalsociety.org