Wow; what a problem. But there are a half dozen good solutions anyway. Much good advice has already been given by those who've already commented. You are in Rochester N.Y.. So that means you are subject to both deep frost, multiple freeze thaw cycles, and snow loads. You need to consult local building code, or N.Y.S. building code to determine at what depth below the ground the footings are required to be placed; i.e.probably somewhere between 36 to 48 inches below grade. This is what originally should have been done, and this is what should be done now as your cure, so that "heaving ground", due to frost, will not create the same settlement problems in the future. (Also, without doing this job according to code could hinder you from gaining other permints in the future, or from gaininging a C.O. on the premises if you ever decide to sell.). Doing the job is not at all difficult; i.e. only requiring time, materials, a plan to do it right,(according to code), and to work safley, as you are working around in improperly built structure. If the wooden part of your structure was built properly, then you could procede as follows; break out the existing garage floor a section at a time, supporting the outside garage walls as you go. Then, in preparation for excavation under these outside walls, support the garage under the sillplate at eight foot intervals with 6 or 8 ft long beams running through the brick foundation, and being perpendicular to the garage walls. Once these are in place, level the entire sill plate by adjusting and supporting the ends of the inserted beams as your way of leveling out the entire structure.. Starting in one corner, excavate down and build the footings, and then then build foundation walls up to the bottom of the already leveled sillplate, (The proper dimensions for both your footings and foundation walls will be spelled out in a readily obtainable N.Y.S. Building code book, available either from your loacal town,county or state building dept. offices.).(when complete in one section, then excavate and build the footings and foundation walls in the next section, do the next, and the next, and the next, etc, until completion. Before you set the sillplate on the finished foundation walls you might consider putting a secondary, treated sillplate in place with your termite shield. Now, as for the garage floor, before replacing it, you'll probably want to prepare the subgrade by compacting it with a vibratory compactor,(this will reduce the future possibility of cracking due to settlement of the ground beneath.). Finally, with your grading finished, place a visqueen or plastic liner on the surface prior to pouring your concrete floor. (The plastic sheets acts as a barrier between the concrete and the earthen subgrade so that water is not lost from the concrete before it has a chance to properly cure.)... regards; der swede