Mitch:
Lauren's answer below is spot on. Particularly when dealing with your grandparent's generation, a carefully hand-written document is so much better (and less threatening) than a phone call, email, personal visit for opening the relationship. Be sure to include a simply drawn diagram of your ancestry that includes you, you parents and your grandparents (including the great aunt) and include birth, Marriage and death dates on the chart. You might also include copies of any photographs, etc. that you may have.
Be sure to explain that you are looking for nothing more than to learn about your roots, your heritage. These are warm words that resonate with the elderly crowd.
Good luck and thanks for sharing here and on WikiTree.
Keith