Looks like cleaned-up rust-pitting. Not quite in the tooth root, but if luck is not on your side and your engine has more than stock horsepower, it's a guess as to whether that will become a broken tooth or not. The Ford/ZF steel is not normally considered weldable, either. You've been trying to decide whether to change ring-and-pinion gear ratios; this is an excellent reason to do so.
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