Go to the NelsonPerformance.com website and there is a security relearn procedure on there, if I remember correctly. Not sure if its the same for the GMT900s though.
One way to reset it is to close the door, lock it with the remote, set for 30 secs, unlock with the remote, insert key and start truck. See if the light goes out after that.
When you say spare ecm/tcm are you saying they are your factory originals, unmolested?
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