Ed, Do you happen to have any images that would show an installed aux. box? I am curious about what you refer to as rubber mounts. I thought I would just mount metal to metal? I agree, it looks to me like the forgiving mount would be the front mount whereas the rear mount looks to hold everything stable and does not allow for the drive-line torque to move the box. I looked at another aux box that had one of the rear mount ears broken off. I am sure due to over torque and probably a little looseness in the mechanical mounting between the frame bracket and the aux box.
Alan