I have a Unimog FLU-419 and yesterday I moved it a couple hundred yards to do some work and it went forward fine. When I went to back up it would not go backwards. I put it in reverse as I always have but it would not move. I then put it in forward gear and it would not move forward either. I've tried everything - low range, high range, reverse and forward in low and high range, and everything seems to shift fine and the clutch seems OK but it just won't move in either direction. The PTO works OK and I used the backhoe some to do the work I needed to do and I moved it into place using the backhoe but it just won't go forward or backwards on its own. Does anyone have any ideas as to why this Unimog won't move?