Is there a way to install a headphone jack in the audio system of a 1997 4Runner Limited stock radio/CD (single disk) system?

Yeah there is. You have to splice a speaker output and put some type of output controller on it, but it can be done. This isn't stock or even an option from Toyota because of the dangerous atmosphere that it promotes. With your ears covered my earphones, you cant hear other cars or emergency vehicles. Its also illegal in many states to drive with earphones on. Just be careful...