Sort Search Results by "Last Reply?"

Unless I'm missing something, the only time-related sort option is by last update, which can bring up anything from one day to two years.

Is there a way to add an option to sort results by the last time the thread was replied to?

Unless I understand you incorrectly, it sounds like you are being redundant. They're the same thing. By selecting 'last post' (i.e. last reply) under the custom sort, it reverts to the 'recently updated' filter.