Not sure about the backing vocals but according the bonjovi.com q&a that very high singing at the end is Jon.

Question:
Hi, I was curious as to who is singing the high notes at the end of "Love Ain't Nothing But a Four Letter Word"?...I think on the dvd Richie mentions that it's David Bryan but I wasn't sure...I think it's a really good song!! Thanks.
asked by Michelle on Wednesday, February 9, 2005
Answer:
No, that's Jon Bon jovi.
answered by BON JOVI INFO HQ on Wednesday, February 9 , 2005 7:00 PM