It won't hurt anything because there is an inner axle seal at the Diff. However as stated above if you do have a warranty that covers it get it fixed.
One thing I would think about though... If you did have it fixed they would have to disassemble your break and hub assembly, drain the diff fluid, and remove the axles. Just to replace those seals. Sometimes it is just best to leave well enough alone. Then like others have said when it is time to do ball joints and other things do it then.
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