Victor Moses then added a second to leave the hosts in freefall and all but end their automatic promotion hopes.
They are now just a point ahead of Palace and will have to rely on their rivals slipping up to have any chance of sealing a place in the top two.
Tamas Priskin should have opened the scoring with a golden chance on 57, but he headed against the crossbar from six yards.
But Palace were ahead on 72 when Ifill brilliantly killed the ball with his right foot, lifted it over Leigh Bromby’s challenge and then smashed a rising half-volley into the net off the far post.
And three minutes later Moses killed the game off when his low shot went through Richard Lee’s legs and snuck into the net.