Well, admittedly, not just everybody who works on cars (which at that,
spans a wide range of training and ability and professionalism,  not
specified by the original poster) is good at setting up ring-and-
pinion, or wants the work.  (You gotta admit, they're messy and
usually don't *smell* real good.)

Righto.  Consider talking to a shop that specializes in driveline
matters.  Usually a 4x4 club or the rod'n'custom crowd -- or a good
auto parts store catering to such folks --  knows where they are, if
you don't want to go in cold through the Yellow Pages or the web.

As for cost, a specialist shop might find that it doesn't need a
complete overhaul, just a specific repair (though unless it's one low-
miler '85 it probably could *use* a thorough going-over).    If you
trust your present mechanic and his taste in junkyard parts, you might
inquire whether the price includes good brakes, etc.  Of course,
whether it's a fair price (my hunch is that it's pretty okay as an out
the door total) and whether you want to put that kind of money into
the car are two different issues.

I'm assuming here that you don't have the experience, time, and
logistics capabilities to either obtain and swap out an axle or do  a
rebuild yourself with safety and quality. The former especially *is*
heavy work on  undercar parts.   However, you might want to know
what's involved as you evaluate your options:
http://www.thirdgen.org/beefinguprear
http://www.chevyhiperformance.com/techarticles/148_0207_10_bolt_rearend_gear_ins
tall/index.html

And just to exercise my sense of the obvious together with my
ignorance of the specifics of your model and year, be sure you're
talking about the same options (limited slip comes to mind) when
comparing prices... and that you've got the same axle ratio on both
ends of your car! (http://www.s10forum.com/forum/f13/rear-axle-ratio-
rpo-12655/index4.html)  I think you had three choices, being 3.42,
3.73, or 4.11, and using different ones at front and rear could make
engaging 4WD on any but the softest surfaces quite interesting,
briefly!

Best of luck,
--Joe