Have you found them in the censuses? If you can find your grandfather living with his parents you can get their names, ages, etc. Some censuses give immigration year and how long they've been married which would help in finding their border crossing into the US. The ship manifest(s) may give area of Germany they are from. Obituaries may supply useful information.
Names aren't always transcribed properly. So, you may need to play with the details you search with.