Match the symptom
The transfer case fluid symptoms are shudder or binding on tight turns, a preventive service around 40,000 to 60,000 miles, and a drivetrain malfunction after neglected fluid. The actuator symptoms add xDrive stuck in rear-wheel drive or reduced power, and a whine or grind near the center of the car. A clunk on takeoff is a different part, the guibo, covered in the BMW Guibo and Flex Disc Replacement. See Transfer Case Fluid (xDrive) and Transfer Case Actuator (xDrive VTG).
Start with the fluid
The notes say the wrong fluid causes shudder, and that the service is often cheaper bundled with the front and rear differential. The front differential page says its fluid often turns black earlier than the rear on an X5 or X7. Intervals and the three fluids together are in the BMW Differential and Transfer Case Fluid Service (xDrive). A whine from the rear, as opposed to a shudder in a tight turn, is the BMW Differential Whine and Fluid Service.
Actuator versus a whole transfer case
The actuator bolts to the outside of the case, and the page says ISTA programming or adaptation is usually required. The important note is to diagnose actuator versus internal damage before paying for a whole case. Uneven tires accelerate wear. The priced reports on that page mix an actuator parts price with quotes for a full transfer case and for programming only, so read the "what" column before comparing. Transfer Case Actuator (xDrive VTG) on a BMW F30/F32/F10/F25 and similar models typically costs $263–$5,700 at a BMW dealer or $463–$1,880 in parts DIY, based on 5 real owner-reported prices.
Cars this shows up on
The fluid page names F-chassis and G-chassis xDrive cars, including the F30, F25, F15, G05, G07, G30 and G01. The actuator page adds the G20. Model pages with priced xDrive repairs include BMW F30 repair costs, BMW G30 repair costs and BMW G05 repair costs. A drivetrain malfunction has other causes, listed in the BMW Drivetrain Malfunction Warning: Causes and Costs. Transmission fluid at the 6-speed connector is a different leak, the BMW Mechatronic Sleeve Leak.