The main difference is that Egoscue is a holistic therapy, meaning it looks at the whole body. For example, if someone has hip arthritis, your posture therapist might find that your shoulder position & lack of function is one of the main causes of the hip.
In most PT settings, they are not going to look at the shoulder or treat it (even if they see it's a problem) because that is not what is being covered by insurance or the referring doctor may not be okay with it.
And since Egoscue is a whole-body therapy, spine issues that you mention fall within that category. The important distinction is that Egoscue does not treat the symptoms, but the overall function of the body -- creating more balance and function.