The "sagging" you refer to isn't all that uncommon, though obviously it shouldn't be there on a home only 9 years young. You say there are no cracks in the drywall anywhere, yet the sag is noticeable both internally and from the street, this would suggest to me that the house was built with the center support (between the garage doors) lower than the walls on either side and the builders and drywall installers worked around the problem. It takes very little movement or settlement for drywall to crack.
"Crowning" is referred to the placement of lumber beams/joists with their highest point along the length facing upward, this could give you your center dip but I doubt it as there would need to be many beams/joists with severe crowning all in the same area.
Your concrete slab most likely is between 3" and 4" thick, this will not hold as a footing for an adjustable post to to lift the sag, you would need to cut out a 12" square section, dig down to code depth for your area (frost line etc...) and fill the hole with concrete then install you jack.
Generally when lifting homes you do it like a tortoise SLOW and STEADY or you WILL crack drywall and have door problems 1/4" lift leave a few days then 1/4" lift....slow and steady.
One thing to look for is how doors are presently closing above the garage, do they close easy?......when closed have a look at the space between door and frame, is it uniform are there points on the door or frame where there have been obvious rubbing, this will help guide you on if it's settlement or if the house was built around the lower center span post on the garage.