"Trusted AD Domain". That's dubious line at best :-)
I've not seen AIX use AD for user auth but I know AD will work for Linux systems.
HOWEVER - The AD admins must add the Linux domain as trusted so it can exchange the encrypted data stream. Alternatively, there's a way to do this without the Linux system having trust in the AD realm. Something to do with not usng TLS.
The RHEL docs on user auth are pretty good for this.
You could try to set up a FreeIPA instance that is in the AD realm and provides auth for AIX. Not sure if AIX has a sssd-ipa package or not. SLES has sssd-ipa but doen't have freeipa server so server runs on RHEL.