Not sure what you mean by kicks out when fires. Assuming the starter disengages prematurely before the engine fires or you let off the key, I would suspect a starter motor is the most likely cause. You can inspect the flywheel/fllex plate ring gear teeth when the starter is removed. If the flex plate is cracked, it will usually make all kinds of horrible noises when the starter is engaged, and possibly a ringing or loud metallic clacking sound when the engine is running at idle.
What yr model and how many miles on your 6.0L? Filling out your trucks signature is helpful.
I had to replace my starter motor a few yrs ago, as it started acting up.
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