This was obviously taken care of long ago.. but just for the sake of satisfying the request:
I took the rear end apart, found a chipped tooth from a gear floating around in there. The chipped off piece of metal was wedging itself between gears causing them to push apart and not engage right away. This should be taken care of, I can get it in before school starts for a full rear end rebuild, or it can just be sold as is since apparently you're getting a new bus anyway.