Not trying to be negative, but I doubt a fluid change is going to fix a noise like you describe. Unless the fluid is low or somehow has water in it; the original diff fluid should last well over 100,000 miles in an open diff. It is always good to get fresh fluid, but without the trac loc clutches it's not something that has to be changed often. If your Tbird has always made this noise, it could well be just how the gears mesh in your differential.
I know you said you are sure it's your diff, but it's not uncommon for our transmissions to make a little noise like you described, at low speeds.
96 Cougar Sport - T45 Swapped