It's pretty easy to disassemble the diff, all you have to do is unbolt both output flange cups. I believe on the small case diff there are 6 bolts on each side. Once both are removed the internal unit will just fall out.
I found an awesome thread on E30tech.com that detailed taking apart the whole unit, here's the link.
http://www.e30tech.com/forum/showthread.php?t=44131 You have to be very careful to keep the shims on the side they came off of. If you are just having the spider gears fixed it shouldn't mess up anything with the shims (I think).