Q: When slowing down to 30 mph, van starts to vibrate

When I am slowing down and my speed is around 30 mph, my van starts to vibrate and all the way too. Three times I have taken it to the auto repair shop and they say it is within Ford spec. But they told me to call Ford customer service, which I have called Five times! The customer service line tells me that if my van is within Ford specs then there is nothing they can do and I’m to take it to a different repair shop, which I did. They kept it all day and then told me that there’s nothing wrong with the van. He told me that all Ford 150s do that. I asked “All vibrate?” He said, “Well no, just put miles on it.” The vibration is so bad you can feel it in the passenger seat. If two shops say it's within specs, then there’s nothing I can do. In my opinion, they might just be saying that since they can't find what is causing the problem. I've been driving Fords most of my life, but that might change! Can you tell me what to do to fix this awful vibration in my van?