I'm surprised no one has mentioned this.
I'm sure you will have in your area a service run by the Department of Agriculture, or the Ag School at one of your state universities. It's called the Cooperative Extension Office, County Extension Agent, or something similar. Look in your local phone book.
Here it is:
Forsyth County > Cooperative Extension Service (ain't Google wonderful?)
They get paid to help with problems just like yours. The Extension Office will be familiar with the soils in your area, as well as what grasses will thrive in your climate. You can take in soil samples which they will analyze for free or at nominal cost. Don't let the "Department of Agriculture" thing put you off. They work with city and suburban folks as well as farmers.
Your problem sounds to me like you may have sod web worm.
An infestation of little critters that live under the sod and eat the roots of your grass. Often associated with too much water and poor drainage, and your back yard looks like it has a bunch of low areas.
The Extension Agent is your friend. And more to the point, you pay taxes to support that service, so use it.
Lindy