The Trak-loc diffs should have the fluid changed semi-regularly, to get the old clutch material out of there. Or when it's taken apart to replace the clutches.
On an open diff, I wouldn't change it. There is a greater chance of getting the wrong type or amount of fluid in, or starting a leak, than there is having something happen because the fluid is too old. My old 95 T-Bird had 220k on the original fluid, my 88 had 206k, and Brenda's old Cougar had 187k. Whisper quiet.
A fluid change on these cars is not fun, or easy with no drain plug. That's why guys pre-fill the diffs before installing them!
97 T-Bird LX 4.6 - 80k miles
94 Supra TT Auto - street/strip car
04 CVPI- Brenda's car - 76k miles
95 T-Bird LX 4.6 - fully optioned, owned 15 years, 220k miles
96 Cougar XR-7 4.6- Brenda's car, owned 11 years, 187k miles
88 T-Bird 3.8 - first T-Bird, owned 5 years, 206k miles