I believe that Dharma is not contained within Buddhism, but rather Buddhism is only one of many that draws from it. Specifically, I believe it's one's openness to be aware the truth within one's surroundings. I don't think Dharma is necessarily a teaching or doctrine to introduce or indoctrinate one into Buddhism. The truth is simply out there and existent with or without our noticing it. Dharma is when we become in tune with it and give it life.
But this Dharma isn't necessarily a path away from whatever belief or religion we currently belong to. I think this treasure – this universal truth – is simply being open to learning. I take Dharma as taking and accepting truth whenever and wherever we discover it.
In fact, this might very well fly in the face of propositional truth. It may very well be the cure for the disease of Fundamentalism. (But again, let's not underestimate the great human (dis)ability of denial).
The Dharma-Ocean becomes the acid test for immutable fundamentalist beliefs. Why do so many people believe they must defend the beliefs they hold to be true? The truth needs no defense. It preserves its identity under any and all changes and challenges. It is unassailable.
"The Dharma is no one's property; it belongs to whoever has the true knowledge" Sogyal Rinpoche, The Spirit of Buddhism, pg. 14