Chevrolet equipped the 2003 Silverado with a transfer case -- either manual or automatic -- on its four-wheel-drive trucks. The shift lever for the manual transfer case is on the floor to the right of the driver. If you have an automatic transfer case, buttons to shift into and out of four-wheel drive are located to the left of the instrument panel cluster. The manual transfer case takes Manual Transmission and Transfer Case Fluid, part No. 88861800. The automatic transfer case takes Auto-Trak II fluid, U.S. GM part No. 12378508.
According to the owner's manual, your Silverado -- if it has the manual transfer case -- should take Dexron-III automatic transmission fluid. The change to Manual Transmission and Transfer Case Fluid was specified in service bulletin No. 04-07-30-037E released in April, 2011.
