A rack mounted to a axle is more common than you would think and actually works pretty good. No chance of bump steer with the rack mounted to the axle as they always stay in the same relationship to each other. The key is using a u-joint and splined slip shaft that allows the steering shaft to expand and contract slightly as the axle moves up and down as the suspension works. I have a few friends that have run racks on axles for years and I haven't heard any complaints other than that they don't like the looks of the rack hanging off the back of the axle.
