Hello Brett ...
Having been a moderately heavy drinker myself in the past, and more importantly, drinking alone in the evenings ... I would believe that whatever depression your wife may have is not likely caused by the drinking - but the drinking is likely a manifestation of the depression. And, of course, drinking is a temporary escape - from loneliness, self-esteem problems or almost anything else. I think the source of the depression is best determined by a counselor or psychologist. I also think that if there was some sort of chemical imbalance (that might best be solved with medication), it would have been visible long before three or four months ago. That too, should be easily identifiable by a psychologist.
I stopped smoking three years ago myself, and with it came a huge reduction in drinking. I thank God ... that I was not or am not an alcoholic. I was doing this for many years - since shortly after my divorce over 20-years ago - so alcoholism was a concern of mine. I am not suggesting that it might be for your wife.
I know from personal experience, that convincing someone you love to go to a counselor (maybe a less-threatening title than a psycologist) could be difficult. It might be less threatening to suggest going to a couples counselor - but please try to find some professional help. We all need it from time to time.
The depression could be something fairly simple, not at all obvious to you, and fairly easily resolved ... and it's likely related to some lack of ability (or desire of) communications - so the trained counselor is the best place to start.
The best to you and yours