Chock your wheels well so that it can't move when pulling with the backhoe.
Ideally, you change your technique so that your using the bucket curl to do the work of breaking things loose. Then, you are only filling the bucket with the spoils you have loosened, as you pull the dipper in towards you. This eliminates pulling against the tractor as you dig.
It doesn't take long doing this, before you can dig all day without dragging the machine.