They are the same. FYI, I seldom heard 서가 in a conversation. 책장, 책꽂이 is much more common.

"책장"이랑 "책꽂이"라고 아주 더 들어요. 이거는 "서가"라고 처음 듣기 있는데요. 그렇지만 "책꽂이"도 "shelf"말이고 "a case for books" 말이에요. "책장"이라고 "pages of a book" 말이에요. "서가"는 조금 설명하기 어려워요. 죄송합니다. I hear 책장 and 책꽃이 way more, this is my first time hearing, 서가. Although that may be, "책꽂이" means, "shelf" or "a case for books." and "책장" means "pages of a book". "서가" is a little harder to explain. I'm sorry...

Might be better to post this in Korean rather than English