That is not a recent crack. If you look at the trunk, you can see that it sort of folds inward by the crack. That indicates the crack has been there for a long time and the other parts of the trunk have continues to expand at a greater pace. It most like is a crack caused by sunscald. That happens when a young tree is not protected from winter sun and it expands when the sun hits it and the shrinks when it gets cold at night. This process repeats and a crack forms. You can tell is there is rot going on inside by taking something like an ice pick and sticking it into the center of the crack in various places. If there is little resistance, then the inner wood is rotting. If you cannot get it to penetrate, then the core is probably still good. The liquid looks like Wetwood. I will give you a link to that. You also may want to have a certified arborist come out and make an on-site diagnosis. A mature tree like that is valuable and you don't want to take it down if you don't have to. On the other hand, if the tree is rotting internally you probably should take it down before it becomes a hazard to people and property. The link to Trees are Good will let you find a certified arborist in your area. Here are the links: http://www.treesaregood.org/
http://www.extension.umn.edu/garden/diagnose/plant/deciduous/hackberry/branchesdiscolored.html    

Thank you. Very helpful information.