I had the same problem after only running it a few times not hard in the grass.  I removed the rear diff and took it apart only to find out the gear was stripped. [:@]  I contacted redcat via their support about this and they told me that it was not a good idea to run it in the grass. I replied back and said that's BS. For the cost of this truck this should not happen. So, I replaced the gear and know it just sits on the shelf.