try american muscle for the gears and for the t-lok scrap that and get a locker or true trac from e-bay if lucky or a website.
also get the install get at American muscle its like 75 bucks there i think...but how many miles does your 04 have you might not need it. also you will need 3 quarts of diff oil my gear with install ran like 460 parts and install

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2004 Redfire Mustang V6-T5-bolt ons and a gear-14.5@97 (SOLD...never forgotten)
2003 DSG Mustang Mach 1 TR-3650-IUP-(BUILD DATE
02/28/2003)-13.4@107 new times coming soon