Hoping someone can identify the name and author for me:
MC is military medic (E-6 or E-7 I think) and is home on leave between duty assignments. IIRC he is upset that he didn't get the assignment he wanted. While on leave, he decides to go camping/hunting for awhile in a remote valley. He ends up about 150-200 or so years back in the past.
He helps the local Native Americans he runs across and sets up a small homestead there in the valley. IIRC he provides medical treatment to any who need with the understanding that his place is a neutral area to the warring tribes. Later, he finds an old girlfriend of his that had gone looking for him when he went missing and managed to come through the same temporal anomaly he did entering the valley (the temporal anomaly isn't always there and barely visible as a slight shimmer in the air when it is active). She tells him that he has been declared AWOL by the military. They decide to stay in the past and start teaching the Native Americans better medical practices.
So I remember the story fairly well even though it has been a few years since I read it...but can't remember author or name of the story.