How do I search SourceTree across numerous commits to know which JIRA ticket a change merged into the dev branch was made in?

Across the bottom where there are tabs for File Status, Log / History, and Search pick Search. At the upper right where there is a dropdown for Commit Messages, File Changes, or Authors pick File Changes. In the large long search bar immediately left of the dropdown, search for a magic string which should only appear in your particular commit.