Have you eliminated the glow plug circuit as the problem?
If the glow plugs are not working properly, it can and has often caused a no start condition.
What is the battery voltage when cranking? Slow crank?
Are you running biodiesel? We've had a few trucks where the fuel sock in the tank had become clogged. They don't recommend cleaning them, but for the $500 price tag of the pump assembly, we've cleaned them with some success.
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