I was recently in two garages in homes that where built about 10 years ago and both had cracks in the garage ceiling. The ceiling joists of your garage, l do not have enough support to compensate for the weight of the drywall or storage in the attic. In fact, most ceiling joists would, over time, sag under their own weight. The ceiling joists serve two purposes: To keep the walls from spreading apart under the weight of the roof and to support the weight of the ceiling.  So try this to solve the problem. First remove any stored items in the attic. If the ceiling still has a major sag, build a two-by-four wall in the garage to lift and brace the center of the joists.