wheel woogie woes

Got good news!! The bf found out he isn't going to have to work till tomorrow night, so he is going to be able to take me to the junkyard. One of the guys at school owns a Duster and was telling me about one place in SC that has all older cars so I'm going to try to get the address from him today.
Now I have questions...
What should I look for to find an axle?
Are all the 489s the same or do I need to measure, and if I have to measure what are the critical specs?
Are there any other rears that might have an axle that will fit?
What price range should I expect to be paying?
And lastly, if there are any questions I haven't thought to ask that y'all want to pipe in on, please feel free :)

Thanks guys!