Installation kits are essential when installing new gears on your GM 2500HD/3500HD and ensures optimal and reliable performance long term. Our gear installation kits include all the necessary components such as shims, bearings, crush sleeves, and more, offering everything you need for a perfect installation of your GM's front and rear differential gears. Pair your installation kit with our selection of Front Gears and Rear Gears! Â GM Duramax Fitments: 2001, 2002, 2003, 2004 GM 6.6L Duramax.Submodel Fitments: WT, Custom, LT, LTZ, High Country, 1LT, 2LT, Pro, SLE, SLT, AT4, Denali.Some exceptions apply, please see individual product pages for specific fitments.