You give me a royal pain in the neck

Mary Smikin's mother never got along well with her daughter's husband, Ted. But Ted almost kicked her out of the house when the old lady told him, "It's just too damned bad, Ted, that your mother kept you instead of the stork that brought you."
But Ted just about got even with her when they were deciding on guests for a party they were giving. "I think your mother should come, Mary. There's always room for one bore at every party."
When his mother-in-law arrived for the party,  Ted couldn't resist saying to her, "you give me a royal pain in the neck!"
To which he got this reply, "Ted, you give ma a pain, too, but it is about two feet lower!"

1. "to give someone a pain"means to annoy or bother someone. And the speaker say "to give someone a pain in the neck". What's the difference between these two phrases?

2. What does the woman mean by "it is about two feet lower"?

Thank you your time and help!