Why is bark peeling from a branch on my grapefruit tree?

It is quite possible that you are seeing the result of sun scald (sunburn for trees). However, it is difficult to diagnose without a little more information, such as the direction the limb faces, which portion of the limb is exhibiting the symptoms, whether or not pruning or die-back has occurred, etc. Please make an appointment at your convenience, and I will be happy to give you a more accurate answer.