I stumbled across a breed, and the more I read about it, the more I think may be an influence in her genetics. It is the Catahoula Leopard Dog.
I still think there is Great Dane and Boxer in her background based on those physical and personality traits I'm familiar with in those breeds and what I've seen in her so far. But Catahoula would explain some things I've seen in her that don't match up with those other breeds.
Hubby suggested possibly some Staffordshire terrier, based on the dome shape of the top of her head. I've known several American Staffordshires and Staffordshire Bulls, and while there is a minor similarity, its not the same. Besides, she doesn't have that cheek thing going on that is so prominent in those breeds.
No matter what her background is, she is a good dog so far. Leaps and bounds ahead of Shelby in the behavior department, other than some jumping up I need to continue working on with her.
The best thing? She sleeps through the night. Hellby is still staying up until all hours, and/or getting up a time or two at night. It's getting REALLY old!