20+ years of "Building 904/727 for drag racing use"

May I ask a question I am needing answers to soon this spring? About my 60 thousand mile transmission in my van 904 I believe, reverse just stopped working, I have been told it can be fixed while its in the van.
My question is what broke, where is it ? What is it called?

Thank you guys, hope I didn't interfere with any direction on this thread.

Man on the hill in northeast Arkansas, memike

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Make year and model