There are circumstances where they have different meanings. “Exculpatory evidence” is any evidence that might tend to cast doubt on on the guilt on a suspect or defendant. Evidence that exonerates a suspect or defendant is unambiguous – it is more conclusive that the person is not guilty.
For example, for a person accused of rape, a statement by some third person that the defendant was somewhere else at the time is exculpatory. DNA evidence from the semen sample that proves the rapist was not that guy would almost certainly exonerate him.