Can these expressions be used just about interchangeably for all but the most formal prose, or is there a subtle difference to them?
He is headed over to the garage.
He is headed for the garage.
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These are typically used interchangeably.
There can be an inferred subtextual difference, though.
Headed over to can impart an airy quality. It seems to suggest he'll get there when he gets there. (He might stop for a cup of coffee, along the way.) I want to make clear, that this is NOT necessarily implied by this. It is just a feeling you might get upon hearing this phrase.
Headed for implies that he is going in the direction of the garage as we speak.