gmcfan: The 1/2 ton serial number is located on the front, right side of the differential carrier. Example MA 0420 MA = 3.90:1 ratio 04 = Month built (April) 20 = Day built(20th)The ratio codes are as follows: MB,ME=3.38:1 MA,MC,MD,MF,MG=3.90:1 MT,MU=4.11:1 The letter/number codes will be stamped, not cast. Take a flashlight, WD-40 and a small piece of scotchbrite and a wire brush with you. Hope this helps. taskforcetrucks
