GM has been having a huge problem with front wheel bearings causing the anti-lock light to turn on. It's much less common to have a rear one cause trouble. A little normal play develops in them in as little as 15,000 miles but due to the design of the wheel speed sensors, there isn't much of a signal developed to begin with, and that play allows the signal to get weaker until it drops out and can't be seen by the computer.
GM has always preferred building assemblies as opposed to multiple individual parts because it takes less time to put them together on the assembly line. Because of that, you have to buy the entire wheel bearing assembly to replace the wheel speed sensor.
To avoid a lot of frustration, have the car repaired right away. Whenever the yellow warning light is on, the computer will turn the anti-lock function off and you'll just have regular brakes. The problem is when you wait a long time to have it repaired, you're going to be surprised with additional parts that no one was expecting. To set the diagnostic fault code, there is always a long list of conditions that must be met, and one of them is that certain other codes can't already be in memory. In this case, the computer looks at all four wheel speeds and compares them to be sure they're equal. Once a code is set for one wheel, the computer won't be able to use it as a reference to test the other wheels. In the meantime, with the extremely high failure rate of these bearings, it is likely a second one will develop signal dropouts but no fault code will be set. When you finally take it in for repair, there could be two wheel bearings that need to be replaced but there will be only one fault code in memory. The mechanic will only know about that one, and that's what they base their repair estimate on.
Once the first bearing is replaced and the fault code is erased, the computer can start doing self-tests again, and the new code shows up on the test drive. The mechanic is frustrated because he has to start all over with the diagnosis, and he has to tell you the car is not totally fixed yet after you spent all that money. You're frustrated because you're sure he didn't diagnose it properly the first time. Most of that frustration can be avoided if you have the first problem repaired right away.