My grandma was telling me about this reoccuring nightmare she has had since she was little?

Reoccurring nightmares might mean shes got to do something and death can be very different. The Big white truck could be a massive boulder she needs push over, maybe a demanding person in her life. The idea of it falling on her is how this person has belittled her, tell her to write down what happens, how it happens and what her emotions are after the dream. This will help you analyze the dream properly. Maybe offer going to therapy if she cant you go. Good Luck. Oh and remember to make her feel better and happier. Move her mind away from these dreams.