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It sounds like the reverse clutch drum is bad and the entire transmission will likely need to be rebuilt. There may be no metal in the pan, but it's probably all in the clutch. The low and reverse clutch are one unit and they lock up to provide engine braking by locking up the sprag clutch. If that whole assembly is shredded it will do exactly what you are saying is happening. You will probably not notice the low gear missing since the A4OD actually only uses the low clutch when engine braking upon deceleration.
Do you plan to attempt the rebuild yourself?
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