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The axle code 3j for the Ford Super Duty means that your truck has a 3.55 limited slip differential. Some Ford trucks may have a 3.73 limited slip differential, which is essentially a 3.75 limited slip.
Unfortunately, the 3J is actually an electronic locking 3.55, not a limited slip differential.
Sorry, I agree with Chris G. I own a 2013 F250 Lariat Fx4 truck with a shift-on-the-fly switch that includes an electronic locker. It has a 3J axle code indicated on the VIN sticker.
3j equals 3.55 and it refers to an electric locking system, while 3L equals 3.73 and it denotes a limited slip feature.
The axle code 3J for the Ford Super Duty indicates a Limited Slip differential with a gear ratio of 3.55.