I will assume that you have GM tranny and not an Allison. There is a solenoid in the trans that runs the lockup function. The usual malfunction, when the solenoid goes south, is the lockup does not drop out. I replaced one years ago in a Monte Carlo SS I had with an after market version for $35, GM want $75 for the same part.
OBTW - The solenoid is accessible by dropping the pan.
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Dale
AKA - Oemy
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