Jerry, none of the things you've said make me feel good about your tree. Its not an insignificant size, and sounds like it has a major defect at the base. Really the best advice I can give you is to have a qualified Arborist inspect the tree on site, and yes you'll probably have to pay for that. Tof find one and to have a chat on the phone visit this site
http://www.isa-arbor.com/findArborist/findarborist.aspx type in your state abbreviation and read down the list of Certified Arborists in your area, you can use the city location too, further down the page.
Having someone on site from your area is the best course of action in this instance.