1986 G20/G25 Chevy Van Tranny question

Trying to help my little brother out. They bought a van , odo reads 52k, assuming it's 152k. They bought it and drove it 300 miles, no problems. Well, now the tranny slips in reverse and doesn't make it to
3rd. I don't do automatics (too many moving parts!). One, what model is it and secondly, is it a band issue? A don't think a fluid flush and filter is going to save her captain!
MTIA, Steve
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If it is full it is done. G25 may be a turbo 400.

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