What to do with a vicious dog--AZ?

We've had a lab/pit mix for a year and a half. She mostly leads toward the pit bull more than lab. She is spayed and current on her shots. Recently, she has become snappy/snarly/threatening to both my husband (God) and myself. We also have a 5 year old boy who can get on her nerves. Apparently she came from a line of fighting dogs. We got her when she was 3 weeks old (really). Our concern now is how to get rid of her. I wouldn't feel safe giving her to another family with her meanness. I really don't want to put her down, because she only gets pizzy when we scold/discipline her. Other than that, she listens well and is a good protect dog. We can't afford special training for her, and she doesn't listen to me. What are we to fo?