You dont say what year , but doesnt engage ?? you mean you dont have reverse?? if not I would say the reverse drum has gone in the tranny either a rebuild or another tranny that simple im afraid, if its intermittent then it may be an electrical issue which only diagnostics would find.
Good luck, Im now in the middle of rebuilding mine and its not for the faint hearted but can be done as all the parts are available all over the world.
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