Jason, I don't know about SD, but my Ex wrote up and signed an agreement much as your's did, when my son was 3. In Wyoming, it was considered a legal and binding contract.
She tried to renig on it before our court hearing, and my lawer informed her of Wyoming law. She freaked out and "kidnapped" (parental kidnapping) our son and took off to Nevada. The scheduled court date rolled around, and low and behold she showed up! (with my son in tow)
The judge didn't even hold the hearing. We went into his chambers, he simply gave me my son, and sent her packing!
One other thing, no matter what, no matter how hard or petty it seems, get a notebook and DOCUMENT EVERYTHING! It will ensure your victory in court - 'cause I guarantee she isn't keeping track of anything. If she is late for visitation, or doesn't want to come, or wants to do something different, etc. WRITE IT DOWN!!!! No matter how insignificant it seems.
Good luck buddy! I have had my son for 12 years now.... IT CAN BE DONE!
-Moto