You need to lift the rear end and place on jack stands. Then looking at the back of your differential you'll see a plug to the right with a 3/8 plug that a ratchet and extension will fit. You just need to untighten that, then fill with the right gear oil. You'll either need to get a siphon pump to go from the bottles to the rear end or you can gravity feed/squeeze it will a long tube from the bottle to the rear end. If you need more, let me know.
A lift though would be even better.
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Help-I need to add oil to my 2003 Mustang Mach I Differential 
