As someone who broke 4 axles in about 18 months a few years ago....
You will need to pull both axles and the diff centre out. Most likley you will need to punch out the splines out of the centre (magnet will almost certainly not work), and check it did not take out the side bearing as they tend to tear as well as Shear and expand as they tear through the splines within the bearing.... Some have a weaker (narrower) point machined into the axle just back from the splines, and if Sheared here your centre will be fine)
Make sure you remove all the oil from the housing as there will be little bits of metal on the bottom of the housing. Replace your diff oil too... (or filter it through a coffee filter if a cheap unpleasant individual like I was and the oil is relatively new)
Providing your centre has not been damaged, it should all go back together nicely and quickly... Try to use an axle that came from the same side to replace teh broken one, as they are stressed over time to a certain twisting and putting one in a "wrong" side will untwist it and it is weaker then...