Can I play Skystrike Squadron for its reduced cost if I target a Shepherd's Heirloom that is attached to another unit I control?

Yes — you can return an attached Shepherd's Heirloom to your hand to pay Skystrike Squadron's additional cost, and Skystrike Squadron then costs [3] instead of [5].

Attachment doesn't change the fact that the Heirloom is still a friendly gear on the board, so it's a legal thing to return, and the unit it was on just loses the gear (it detaches and goes to your hand). Note this is a cost, not a target — Rule 355.10.c — so nothing about the unit it's attached to restricts the choice.

One thing that does not happen: the Heirloom isn't "played" again when it's returned, so its "when you play this, gain 1 XP" won't trigger off this.