As long as the gear oil is still coating everything, (and like mousetrap said, it's pretty clingy) you'll be fine just putting the cover back on. It won't rust. I don't think I'd spray it with WD though, as that would probably break down the gear oil. Also stuff a couple of oily rags in the axle tubes if you have the axles out.
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1985 Mustang GT
'85 GT 13.20 @ 105.8 / '72 Mach1
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