Are you sure it is the adaptor leaking and not the t-case? The adaptor has a female fitting on each end. The t-case has a double ended male adaptor screwed into it. If that double male fitting is loose, then it will look like your speedo cable adaptor is leaking when it isn't. Plus, there isn't much room for too much to even be in the adaptor much less leak for a long time.
Crawl under there with a light, something pointy to scrape with, glasses for your eyes and I think a 1" open end wrench. Scrape the t-case side of the reduction gear clean and see if you can make out the double male fitting. Try to tighten it up. I had trouble holding it the last time I put mine on, so I loosened it up enough to get it tight to the gear reduction unit and then tightend them together to the t-case.
