Doesn't sound like the knock sensor is actually bad, but then again its always a possibility. It only does it when it's warmed up then? Does it do it all the time when the truck is warm, or only right as it's coming up to temp?

If you need a knock sensor though, i took a chance and picked one up off ebay for about $35 shipped a few months ago. It was used but according to the place it was tested and worked fine. So far I haven't had any problems at all. If you want i can find out what the name/contact info was for the shop. I would check that all the other sensors are good and within specs first though.