Quinn's 3-pointer with 6:47 left ended a 4:20 scoring drought and sparked an 11-3 run to give the San Diego State its largest lead at 56-41. The Aztecs (26-7) face fourth-seeded Wyoming in the championship on Saturday. San Diego State swept the season series with the Cowboys.
Colorado State's scoring and rebounding leader, J.J. Avila, sat out with an ankle injury sustained from Thursday night's quarterfinal game. Avila scored 53 points in the two regular season games against the Aztecs this season.
Shepard scored half his points on four thunderous dunks, the last making it 41-27 to cap the Aztecs' early second-half surge.
Dwayne Polee II scored all 12 of his points in the first half, including two 3-pointers, to help San Diego State build a 24-18 halftime lead.
Stanton Kidd led the Rams (27-6) with 10 points and eight rebounds.