Actually they have no differences in spoken Chinese.You can both say "li jie" and "ming bai" when you understand, and you can both say"wo bu li jie" and "wo bu ming bai" when you don't understand. But "li jie " seems more formal, especailly the word is not used alone. There is a special situation that people can only use "lijie" not "mingbai" is that " xiang hu li jie".We cannot say "xiang hu ming bai".The meaning of "xiang hu li jie" is "know each other".You can view it as a fixed usage. The same as "xiang hu bu li jie" .Another difference is that "li jie" can ＋somebody，but "ming bai" can't. For example,you can say" wo li jie ni",but you should not say "wo ming bai ni".Both "li jie " and "ming bai" can ＋something. For example, "wo li jie ni de yi si"＝"wo ming bai ni de yi si",and the former one seems more formal.