The Dukes (18-14), who beat the Tribe (13-17) for the third time this season, face Delaware in Sunday's semifinals.
James Madison, up 34-33 at halftime, but later led 56-49. The Dukes were up 66-58 on a layup by Rayshawn Goins (13 points). But Marcus Thornton, who scored 25 points, hit a 3-point basket, two free throws and a layup to help cut the lead to 70-67 with 46 seconds left.
Thornton, however, missed two 3-point attempts, sandwiching Ron Curry's two foul shots that ended the scoring.
James Madison outscored the Tribe 40-26 in the paint.