Thats a 7.5".
If my memory serves me. G 87 is open diff. I want to say the Y 56 is 3.42 gears.
not sure what the Y54 would be. There is an r p o code break down on g n t type.org. 1986 came with a 8.5 rear end I don't think they made them any other way , but I am learning new things on these cars all the time
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