There are buses that run directly from one major park to the others that are not on the monorail line. I know for a fact that we rode directly from AK to MGM on our last trip.
If by chance I'm wrong the other alternative with MK is to ride the monorail or walk to the Contemporary Resort and then catch a bus from there to AK or MGM. Or do the reverse: bus to the Contemporary from AK or MGM -> monorail or walk to the MK.