Shoulders/legs enchants require you to be level 85, so you can't use it on your alt's heirloom and apply it if you're not level 85.

You can bypass this restriction by applying it from your main character level 100 and mail it to your alt.

Here are the steps:
Create the shoulder/leg on your level 100 toon.Apply the enchant.Send it to your alt. ALSO: If your heirloom is upgraded to level 100, you will need to create it from a character below level 100, because the ilvl of the heirloom is 605 and the enchants are restricted to below ilvl 600.

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