I also agree that chaining to the axles and having the chains deflected by the tires isn't the best possible way, HOWEVER, it is better than some other ways. I would consider using a piece of carpet or rubber mat to keep the chains from stripping all the paint off of the axles and tie down points.
Egon,
The bucket on your backhoe pictures, while tied down nicely, doesn't appear to prevent side to side motion as well as it could. You would be better to use a chain to each side and not have the chain that goes all the way across that the chains that actually hold the bucket are attached to. The two shorter chains will allow the bucket to move side to side a lot sooner than if you had just went straight to the bucket from each side. I figure you probably did it that way due to the amount and length of the chains and binders you had available, and I understand that, however since we are trying to help with "the best way" I thought this worth mentioning.
David from jax