It is best to have an arborist take a look at the tree and see what they think. We were faced with a similar problem a couple of years ago, and decided to remove the tree because a windstorm had cracked the trunk. It could have been bolted to reinforce it but the arborist predicted it would have rotted out from water entering the crack in a couple of years, so it wasn't worth the expense. We hated to loose the tree though because it shaded the West side of our house, which needs the shade in the summer time.
This website has some good information on what to look for to see if your tree is sick.
Save A Tree - Home
Here are some others that might help.