The easiest way to destroy your differentials in your Subaru is to drive around on different sized tires. Most people recommend around 2/32 maximum tread difference between all four tires. A great way to have to replace all 3 differentials is to just 100+ miles on a tire that isn’t the same height as the rest of your tires.

This is why you carry a full spare or if you have to have a AAA membership.  I would be weary of driving 30 miles on a spare that I know is the correct size for the rest of the drives never mind 100+ miles.