If your Ex-Wife was the "Custodial Parent" of your Child during 2017, you will not need to sign anything or jump any additional hurdles. The IRS defines "Custodial Parent" as the parent who has custody of the child for the greater portion of the year, even if only by 1 day.
However, if you were the "Custodial Parent" during 2017, you will need to complete Form 8332 in order to release the Child's Exemption to your Ex-Wife (and her new Spouse). She will attach this Form 8332 to her Tax Return.
Keep in mind, the Child Tax Credit can only be claimed by the Parent claiming the child as a Dependent on their Tax Return. For more information about the Child Tax Credit check out the TurboTax FAQ below:
https://ttlc.intuit.com/replies/3301702
