Thanks for using the forum. I am happy to address your questions, and my answer will be based on the information you provided here. Please make sure you recognize that this forum is for educational purposes only, and it does not substitute for a formal office visit with your doctor.
Without the ability to examine your wife and obtain a history, I can not tell you what the exact cause of her symptoms is. However I will try to provide you with some useful information.
I am not sure exactly what you mean by slurring of words. If her speech is normal in content, her language capabilities are OK but her articulation is not clear, the medical term for that would be dysarthria: poor articulation basically. There are many possible neurologic disorders that can cause dysarthria. However, please note I am not trying to imply with the following that I think your wife has one of these but rather just providing general information. Examples include disorders of the brain, (such as a tumor in the brainstem or a part of the brain called cerebellum), disorders of the muscles in the mouth and throat (such as a disorder called myasthenia gravis), and certain disorders that affect the motor neurons (such as ALS). A stroke can cause slurring of words but this would be of sudden in onset and non-progressive, meaning that it would not continue to get worse over the months.
The majority of disorders listed and other disorders that cause dysarthria would most commonly be causing other symptoms as well if this has been going on for sometime. Depending on the disorder these additional symptoms could include double vision, difficulty swallowing or chewing, weakness in the limbs, etc.
It is difficult for me to provide you with more specific information, but this does warrant evaluation by a physician. These would be diagnosed with lab tests, tests of the muscles and nerves (EMG), or MRI brain depending on where the problem is thought to be by the physician who evaluates you. Speech therapy to improve the articulation may be beneficial to your wife as well.
Thank you for using the forum I hope you find this information useful good luck.
Your wife may have suffered one or more small strokes. But I would get her to a doctor as whatever it is, it's getting worse!