IT'S MARCH 31 and the revitalized New York Knicks are in Cleveland to face the equally upstart Cavaliers. The stakes are high. New York, at 44-33, sits four games behind the Cavs in fifth place in the Eastern Conference standings. But tonight's showdown offers more than just another checkmark at the end of a long 82-game schedule. The Knicks, who at this point have already won two of three over the Cavs this season, have a chance to send another message to the team they are expected to face in the first round of the playoffs a few weeks from now.