I would go for Upgrade to Nikon 60mm f/2.8D. That one works good with Kenko 2x 300 PRO telekonverter. Really nice setup that I loved with my D300s. The Nikon 60mm f/2.8AF-S don´t focus with kenko telekonverter 2x. And it also feels more plastic than the Nikon 60mm f/2.8D.
Have you tried the teleconverter with the non D 2.8? I'm not sure why it couldn't work with it.