Next up for United will be an intriguing encounter against the Houston Dynamo.
In a game postponed by 24 hours because of a blizzard in New Hampshire and after the first leg finished 1-1, the Red Bulls missed a wonderful opportunity to win the tie when United goalkeeper Bill Hamid fouled Kenny Cooper in the box in the 69th minute, earning a red card.
Cooper scored the penalty but the goal was chalked off as referee Mark Geiger ruled Thierry Henry and Tim Cahill encroached in the area - and substitute keeper Joe Willis saved the retake.
The numbers were evened up when Rafa Marquez earned a second yellow in the 74th minute, and DeLeon decided the match late on when he latched on to Robbie Russell's through ball and finished low past Luis Robles.
Meanwhile the Seattle Sounders made it through to the Western Conference final as Mario Martinez scored the only goal in the 81st minute at Real Salt Lake.
Seattle, who drew the first leg 0-0, will next face the Los Angeles Galaxy.