Thanks for all that info.

So... perhaps I can't afford to do this right now:

Carrier ($419.95) - http://www.summitracing.com/parts/STR-R542060/
Ring and Pinion Gears ($258.69) - http://www.summitracing.com/parts/MGR-BP882355/
Ring and Pinion Install Kit ($87.99) - http://www.summitracing.com/parts/RAT-311K/

Are those prices reasonable?! Presumably I'm also going to need a shop do it, or get some special tools? Any other options? I wouldn't mind taking bits from a scrap car, but that's not really an option over here.

Is this going to be possible to do with the axle under the car, i.e. at a later date?