What is the most likely cause of transfer case shudder at low speed?

The most likely cause of transfer case shudder at low speed is a bad viscous coupling. A viscous coupling is a device that allows for torque transfer between the front and rear driveshafts in all-wheel-drive and some four-wheel-drive systems. It operates by using a viscous fluid that thickens when heated, allowing it to effectively transfer power to the necessary wheels when traction differences occur.

When the coupling becomes faulty or degraded, it may not be able to effectively transmit torque, leading to shuddering or jerking sensations, especially under low-speed conditions when the system is trying to engage. This can occur as the viscous fluid may no longer provide adequate resistance or engagement between the two driveshafts, resulting in a lack of smoothness during low-speed maneuvers.

While a damaged planetary gear set, a loose drive chain, or spalled bearings can all contribute to noise and performance issues in a transfer case, they generally do not produce the same shudder effect at low speeds that a failing viscous coupling would. Thus, the presence of shuddering specifically points to the viscous coupling as the most likely culprit in this scenario.