If you've ever gotten stuck in your 1982-2000 GM 2500HD/3500HD and needed 4-wheel drive, then a transfer case is what helped get you out. 4-wheel high will maintain the same gear ratio as 2-wheel drive and is for high-speed situations such as driving on dirt or snow-covered roads. Bolted to the back of the transmission, a transfer case has two driveshafts connected to it that run to each axle. It is responsible for transferring power to the front wheels and rear wheels in 4-wheel drive systems. 4-wheel low is for low-speed situations where maximum torque is required such as driving up a steep rocky hill or pulling out a stuck vehicle. There are usually two different gear ratio options with a Transfer Case, 4-wheel low and 4-wheel high.
It is highly recommended to not engage the transfer case for anything other than off-road or low-traction situations. XDP carries a wide selection of Transfer Case products available from Dorman, Merchant Automotive, PPE, Sun Coast Converters, and more. A Transfer Case typically does not require internal upgrades or modification unless the vehicle will be used for extreme off-roading or racing applications. In some applications an external brace can be added to increase the structural rigidity of the transfer case to prevent damage from high torque loads or twisting. When you shift the Transfer Case into 4-wheel drive, it locks the front and rear axles together so that they spin at the same speed. Wheel slip is possible in loose terrain, alleviating any binding that can occur on dry pavement.