A young woman on Ask had a similar problem. She found that the Mormons hang on to their own tenaciously and it was very hard to get them to listen to her.
Your grandparents may not like your decision to follow your own reasoning but if they have gathered experience with their years they should certainly respect your decision. Just tell them.
I wouldn't worry about being "technically" a Mormon. You either are, or you aren't. You can't have it both ways. But take courage from knowing this: Of all the Christian sects and spin-offs, Mormonism is among the silliest.