Your rear drive utilizes Haldex to engage/disengage power to the rear wheels. What is common, all too often, is that the oil and filter never get changed and the oil turns into sludge. Another problem is that the computer board gets fried from the heat produced from the sludge.

Yes, a new Haldex unit is expensive. The only options are a new Haldex, or a rebuilt unit from a junk yard car. The rebuilt unit must have the same part number so it can communicate with the car's main computer.
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Keeping it running is better than buying new