To clarify, if the spring is broken at the center pin, and walking out, that will leave the u-bolts that hold the axle to the spring loose, and that is no good. If the spring is broken outside the U-bolt area, and therefore the U-bolts are staying tight and holding the axle to the spring pack like they're supposed to, then you may be OK to drive it. If that is the case, I would jack it up and put the broken piece back in place. Then use a few hose clamps around the spring pack to hold it in. Alternately, duct tape seems to do the trick too. (Hey, I was once a poor broke college kid with two broken leaves and snowplowing that HAD to get done!)
Chris