The short answer is that it just takes that long. Please remember your model year 4Runner, that's covered by this warranty extension, is from 2003-2005. Now think about how many people out there, besides this warranty extension are going to pay for a dash to be replaced on a 10-12 year old car. Not many. There's no need for Toyota to have a lot of these dash pads in stock just laying on the shelf.
Now couple that with the word getting out that Toyota is going to replace your dash pad for FREE even though that your car is 10-12 years old and the warranty ended around 2006-2008. That can be overwhelming for any car manufacturer.
It sounds like you have done all that you can do and now just have to wait till your dash comes in. I can tell you that I have seen a few 4Runner dashes put in recently so they are being shipped to the Dealers. I would recommend that you call the service adviser that originally helped you with this and get a status update. Now do this 1 to 2 times a week and let's see how fast yours shows up.