Before you hire an attorney, you might want to try going back to Fidelity and respectfully requesting that the account be transferred to you immediately, even if your grandmother refuses to sign anything. A quick reading of the Arizona UTMA statute appears to show that your grandmother has no say in relinquishing control of the account to you.
14-7670. Termination of custodianship
The custodian shall transfer in an appropriate manner the custodial property to the minor or the minor's estate on the earlier of:
1. The minor's twenty-first birthday with respect to custodial property transferred pursuant to section 14-7654 or 14-7655.
2. The minor's eighteenth birthday with respect to custodial property transferred pursuant to section 14-7656 or 14-7657.
3. The minor's death.
The word "shall" is important here, but the statute language may be interpreted in Arizona to mean that the custodian must take action to effect the transfer. If that's the case and your grandmother refuses to act, that's when you probably need to get a lawyer involved. Good luck.