I've had a few oddball problems like yours and while the majority tend to be the obvious there are a few which just leave you scratching your head. For instance, on a 2003 with the exact same symptoms as yours the culprit was a bad ICP sensor. It was brand new, but it was also aftermarket (the owner wanted to be cheap), and when I disconnected it the truck suddenly ran without a problem. After further investigation it was showing something like 40 PSI (It's been a while so I don't remember the details) with the truck off and having sat for hours, so internally there was obviously an issue. Checking Vref showed a slight drop in voltage and when I disconnected the ICP the drop went away. Another case (wayyyy different circumstances) was a bad coil on an injector causing my PATS to activate even though those two things aren't related and don't even communicate. My point is there can be some odd things causing problems so just because you don't think it might be the culprit doesn't mean it should be ignored.
If you have any recently replaced sensors which were aftermarket I would try disconnecting them (if you can....obviously the truck needs a crank sensor to start/run). If you can't disconnect then I would first check your Vref with a scanner and then manually with a meter at the sensor's connector. If there are no recent replacements then look at the obvious like Bismic recommended. Try a known good FICM. Maybe even try a known good PCM. It's rare for them to go bad, but it happens. Plus, I'm suspicious anyway since you should be reading some sort of default pressure for the ICP when it's unplugged. The fact you're getting zero is just weird. I'm not familiar with the scanner you used, but I know FORScan, and if you want to take the time to download it onto a laptop or the like you can monitor everything in real time and even record it so you can go back and look, millisecond by millisecond, to see what might be the problem. Perhaps the PCM isn't sending the signal for the FICM to turn on. If so, is the PCM not receiving all the right signals at the right time, or is it just a bad PCM? Monitoring all the variables, recording them, and looking back, will help trace it all down. It's time consuming and can be frustrating, but it's rewarding once you figure it out.