Mitchy,
These steel diffs sound like they are geting pretty scarce, so it is a sellers market. It seems to me that you have got two choices:
1. Get a second hand alloy diff complete with the housing you broke which will set you back about $300 from somebody like
http://www.suzisport.com/ and be gentle with reverse gear in future.
2. Spend the money to upgrade your diff for $900 plus any fitting charges.
So you can buy 3 alloy diffs for the price of one steel one.
Only you can decide how to spend your money, but you might get a heap more wheeling done with a 2nd hand alloy part than what you have over the last few months chasing used parts that are closely related to hens teeth.
Maybe you could get the car on the road on the cheap and keep an eye out for he right part at the right price. Might not cost you more than the $900 you have been quoted and you'd be back on the road.
Cheers