When I got my jeep from my dad, I know the T-case fluid had not been changed in at least 80K miles. I changed it and.... it was clean and not burnt. My fluid has never come out dirty or burnt, a t-case in good shape doesn't generate much heat or particulate.

I would believe he rebuilt it and messed something up. Now there is a ton of friction burning the fluid and getting it dirty. Since it is already out, disassemble it and inspect it. Maybe it never was rebuilt and that is the factory oil. Either way no damage, and you got a dirt cheap case.