For your broken bolt try this thread.
http://www.peachparts.com/shopforum/showthread.php?t=269207
I ended up using a dremel to grind the bolt down and pulling it out that way. Roy posted the part numbers for the replacement metal. He mentions it in there, but there is a small cover that once you remove it can let you access the piece of metal.
If you need to drive in the mean time, you can bolt up the differential with only three bolts. I drove from January to mid July with this. Just remember to get Loctite on the bolts and apply 25 nm of torque to a set of brand new bolts.