My wife and I live in NC and have been having a problem with the discoloration and some slight hairline cracking on the ceiling of our garage. During the summer it is extremely humid in the garage and I have noticed there are a couple of long thin strips on the celing that are slightly discolored and there is a slight hairline crack/seperation in the area that is discolored. It looks like this area is where 2 pieces of drywall join together. I can tell its not damage from a leak since there is no pipes above the area and its downstairs(so no leak from the roof). Is it possible this is being caused by the high humidity in the garage?

By your description you have a bi-level/raised ranch with your garage in the "basement" because you say your garage is downstairs.
If this is right, then I can see it being from high humidity especially if your garage is partially "under ground".

If the drywall seams are cracking this is usually cause by the floor joist swelling due to the humidity. The discoloration probably appears where each wood member is. You can measure them, they should be 16" or 24" apart.

I don't see this a posing any structural problems.
Just do be aware of any black mold growing on the drywall due to the humidity. I'm assuming there isn't any because you didn't mention it.

I'm a remodeler in NC. Sheetrock in garages never works good here, but they use it anyway because it's cheaper. Humidity is a factor, but mostly the cracking is due to temperature change. Sheetrock needs to be used in a temp controlled environment. Unles you intend to heat your garage you will probably continue to have cracks. They are no danger, just unsightly. However if you begin to see nail pops or separation you may want to consider covering or replacing with plywood or a similar durable material.