Would telling them about the scientific studies conducted about LD help, you think? I only have this link, but I think there are more around the forum:

http://en.wikipedia.org/wiki/Stephen_LaBerge

I never had that problem but I know how you feel - my mother is the kind of person that cannot conceive anything that isn't in front of her eyes. Her lack of imagination vs my constant wondering about all things was always an issue between us.