I'm just posting this because I couldn't find anything anywhere about this particular problem.
2002 Tahoe, 5.3 Flex. Driving down the road at 75+ uphill and the engine just quit. No sputter, no CIS lights, no nothing. It was just like someone turned the key. I had 50lbs of pressure in the fuel rails, spark, no codes on the OBDII, checked every fuse and relay. It would crank just fine but no fire (and I mean nothing, no sputter, catch, nothing). There was no pressure fluctuation while it was cranking though. Changed the CKP (crankshaft position sensor) and problem solved.
A note though on changing it though. The sensor is located behind and above the starter on the passenger side of the engine. You have to pull the starter to get to it. Do yourself a favor and remove the oil dipstick and oil level sender before you start. The dipstick is held on with one bolt on the head, then it just pulls straight out. Two bolts hold the starter, one more bolt holds a small plastic shroud, and the starter's out. You don't need to disconnect any of the starter wiring, just let it hang and you'll have access to the CKP. One bolt holds it in place, then it just pulls straight out. As the manual says, assembly is the reverse of disassembly.
Hope this helps someone.