What is the biblical basis for a post-tribulation rapture?

I am fairly well-versed in premillennial theology, however, I was challenged frequently while at Bible College on my point of view of a pre-tribulation rapture in favor of a post-tribulation rapture. I understand the principles of post-trib thought, but I fail to see any scriptural evidence for it. Can someone please point me to the scriptural evidence used to support a post-tribulation rapture? I have read "Gospel of the Kingdom" by George Eldon Ladd and I enjoyed it, but I suppose that perhaps I did not properly understand it or his argument for a post-trib rapture.