Looks like Makerbot fixed the Delrin plunger problem with a upgrade kit! Watch the video below to see how to install it. You can not buy it on their website but you can download it from Thingiverse now! I will update this post with more information when I hear it!
Here is a link to the upgrade on Thingiverse!
Makerbot has now posted a blog post about the upgrade!
Link to the post here!
It looks like they will not sell the upgrade kit for a while, too bad but you can download it from Thingiverse and try it out!
Link to my previous post on what was the issue with the MK7 and MK8 Stepstruder.