Car vibration at freeway speeds; why?

I've experienced a vibration, at speed, three times, in my darts. Each time it was something different.
Once it was indeed the parking break cable, as Memike cautioned against. Adjusting it tighter took a couple of minutes and the problem was solved.

Another time it was after market rims. Even though I had them balanced they still shook your nuts off between 65 and 75 mph, after which you would drive out of it.

The third time it was the front Roll pan (under the front bumper). I had it off for a while and while reassembling the car it some how never got tightened, or came loose. It was still bolted to the car, but not securely. As a result at about 50 mph, it would "plane-out", and start to vibrate. It drove me crazy for about a month.
Finally, I have a couple of friends ride in front of me, and behind me, looking for issues.

Just about the time it started to vibrate, the guy in front of me started wave his hands. We pulled over and he said the roll pan was moving up and down.
Sure enough I grabbed it, and there was about a 1/4" of play in it. I tightened it all down (up), and the problem went away.