Even if you don't live with or near them, you're going to have to tell them if you plan on starting T. (Believe me, it gets to a point where they notice. Very quickly.)
Grandparents can be surprisingly understanding. Mine were. They're fundamentalist Christians, and I didn't think they'd react well, but they came right around and supported me. We showed them that 20/20 Barbara Walters special on trans kids, and I think that helped. That might be something to think about -- it's a very poignant piece and might have an effect on them. I don't know if it's available online or anything, but you might be able to order it...