Your axles are the last link in transferring power from the engine to your wheels. They're strong parts that last a long time – but they can run into trouble.
Axles just wear out over time and need to be replaced. And sometimes axle seals leak, causing the axle to wear out prematurely. Lubricant leaks out, and water and dirt can get in and contaminate the gears. When this happens you might hear strange noises coming from your axle.
If you have a rear-wheel drive vehicle, the sound would be at the rear end. If you have a front-wheel drive vehicle, the sound would be up front. Of course, with an all-wheel drive, the sound could come from either front or back. It might be a groaning sound or clunking when turning.
There are different kinds of axles, each with their own potential problems. Service technician knows this and checks for things like leaking seals, torn CV boots, worn axle joints. An inspection will reveal if there are any problems or if a part is nearing failure.
When there is a concern with the axle shaft, it will be removed and inspected. If it's damaged, the shaft will have to be replaced. If the shaft is in good shape, it'll be cleaned, lubed and reinstalled. Damaged CV boots are replaced as well.
It's important to have your favorite repair shop (hopefully it's us) take care of this work as soon as you become aware of a problem. Waiting only makes the damage worse and more expensive to repair later. If the axle fails completely, it could lock up. This could severely damage other things like your transmission – very expensive. It could also contribute to an accident, which nobody wants.
Taking care of your axles when they need it saves money in the long run, and helps keep you safely on the road.