I apologise for posting here, but as a Mod I can see all sections.
John, I'm probably the same age as your grandparents, I'm mid 50's. They love you. You already know that. If you present as you do in your avatar they already know that you have gender issues. I'd probably guess it has been a long time since they suggested wearing a dress? Parents and second parents love their children with a hope that they will live their lives in a full filling way. We try to shelter our children form the mistakes we have made. No matter how futile that is.
I realise from this forum that there is intense bigotry in some areas of the USA, I'm Australian, but if there is a single way you can demonstrate that you are the man you are, accept it.
I'm a woman so I approached things differently when I came out. You're a man so be male. This is a bit of a cross gender thingy because I'm not sure what young guys do, beyond getting drunk, fighting and vomiting. Sorry.
I would take your GrandParents, who have raised you? to dinner at the best restaurant you can afford. I would dress as a modern young man at a restaurant, even if it is a suit you can rent. Look like the young man you are. Be confident (no matter how you feel) . Be yourself. Be honest and they will accept you. Tell them honestly how you feel and how you are built and how you want to make those changes. Men present stuff pretty bluntly. Present.
Give up hiding. The day you give up hiding is the first day of your life.
When I came out. I told everyone they could ask any question. But I may not answer them.
The questions have been few.
The love has been overwhelming.
Hugs
Cindy