1) Drag this link up to your browser’s bookmark toolbar: Tumblr Reblog
1) Go to the permalink of the individual post you want to reblog (e.g. this) NOTE: Bookmarklet will NOT work on the Tumblr dashboard. If you can’t figure out how to navigate to the permalink post on a Tumblr blog, click on the top-right corner of the post on your dashboard to get to the permalink URL.
2) Click the “Tumblr Reblog” bookmarklet you set up – it should then process and bring you to a compose screen with the post ready to reblog (even your own posts!)
Caveats – I cobbled this together from some existing bookmarklets, so if something doesn’t work properly, let me know. May not work for all types of posts (e.g. ones where reblogging is not supported, etc). Hope this works for you!