Dino
Been there done that,
Snapped a bolt in the rearend
What worries me about this is, thats the bolt that has been replaced due to the previous smaller ones breaking. If the upgrade is starting to break now, thats not good. I'm not sure of the size but I can tell you that you can buy it from McMastercarr.com. I know Cashburn has the size, maybe he'll chime in. If I don't see an answer by tommorrow I'll check my spare one and let you know its proper size.
Also there is no nut. It is screwed directly into the rearend housing which tells me that the other end is still in the housing. Your best bet (and this is important) is to remove the Diff from the car and then try to drill out the bolt using a counterclockwise drill bit. If that doesn't work then you can use an easy-out. Don't do like I did and try to do it in the car and snap the bit off in the bolt...what a headache !