I think that would depend how if you wanted to install a salvage yard rear or just install the ping and pinion and the price of both and what available tools you have.
To R&R the entire axle assembly calls for 3.6 hours. The procedure is, click here.
To R&R the ring and pinion calls for 4.2 hours. That procedure, can be found here along with ring and pinion sets, http://www.drivetrain.com/parts_catalog/ring_and_pinions/#GeneralMotors
Ring and Pinion Installation Instructions can be found here, http://www.drivetrain.com/parts_catalog/ring_and_pinions/ring_and_pinion_installation_instructions.html
Here also, http://www.ringpinion.com/PartsList.aspx?SearchMode=Make&MakeID=3&ModelID=298&Side=Rear&DiffID=45&CatID=10
Looks like roughly $150-$300 for the ring and pinion set.
I called about a rear for my 1990 C1500 and they wanted $700 from a salvage yard for that so that should give you some idea of how much a salvage rear may cost.
If you need any diagrams, here.
Plus they have parts as well.
So does, http://www.gm-partscenter.com/partlocator/index.cfm?siteid=215875
Here, too http://www.gmpartsgiant.com/
Here is another website with instructions on the ring and pinion.
Actually, I found some complete rear axles starting at about $100.
Over 500 results found.
Here, this is the home page put in your zipcode.
At the bottom where it says Postal Code :