If you have your ground cable connected to the axle tube, and you're welding to the axle tube, why would the electricity "arc through the bearings"? If you have the ground clamp on the frame, and you're welding on the axle tube, that would be a different story. They the electricity would have to find a path through the springs and shackles, or through the drive line to get to the frame.
Even so, you'd probably be better off fabricating some kind of bracket that you can clamp to the axle tube for a lower shock mount.
Merle