If its the actuator gear dont let them rape you by replacing the unit. Plastic gear usually goes bad inside, $20 part, order the plastic gear. Jack the car up i believe 4 bolts hold the actuator to the transfer case, unplug the connector and theres a full diy video on youtube how to replace it- doesnt take long nor too much skill/tools.

The bolts if i recall right you need a E type socket external torx head.

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2006 X5 4.8is Fully Loaded