Goals from Luciano Becchio, Jermaine Beckford and substitute Robert Snodgrass took Leeds to their 11th successive home win and they will visit Millwall in the play-off semi-final first leg on Saturday May 9, with the return leg at Elland Road on Thursday May 14.
Watched by their biggest crowd of the season - 34,214- Leeds made two changes, Fabian Delph returning from suspension in place of Bradley Johnson, while Rui Marques came in for Sam Sodje who was rested as a precaution.
Town captain Chris Doig was out with a groin injury but striker Adebayo Akinfenwa was passed fit after a slight hamstring strain.
Delph made a rapid impact on his return with a glorious solo run and shot in the third minute, the ball curling inches wide.
Mark Hughes sprang to Town's rescue just after the half-hour mark with a vital block to deny Beckford after neat passing by midfield duo Neil Kilkenny and Jonny Howson.
Chris Dunn made a fine near-post save to keep out Andy Robinson's vicious drive but Leeds would not be denied and almost immediately they broke the deadlock, Becchio heading home Ben Parker's left-wing cross from close range to register his 18th goal of the season, with Dunn staying on his line.
Aware that scores elsewhere were putting them in peril, Town stepped up the tempo after the break, Luke Guttridge and Leon Constantine going close.
However, Leeds doubled their lead on the hour. Snodgrass, who had replaced Kilkenny two minutes earlier, slid a delightful throughball to Beckford who slotted his shot to Dunn's right from inside the penalty box for his 34th goal of the season.
Robinson blazed over from ten yards but he then supplied a pass for Snodgrass to hit Leeds ' third with a stunning stoppage-time volley.