Differential service kit
Lets the left and right wheels turn at different speeds in corners.
Signs it's worn: Whine or howl that changes with speed, gear oil leaking.
What's wrong with my car? · Drivetrain
Whine or howl from the back of the car?
Usually: Differential.
A whine or howl from the rear that changes with speed on a rear- or all-wheel-drive car points at the differential, often low on gear oil.
The parts that fix it
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This is a first read from the symptom, not a diagnosis of your car. If you're not sure, have a mechanic confirm before you buy, and bring the prices you found here with you.