Re: 1987 SMB in Bishop, CA
You could check wether it came with factory limited slip by checking the door tag on the drivers side. There is an axle code which tells you what gear ratio and whether it is a limited slip differential.
As far as adding an aftermarket one, you definitely can, but it costs much more than $100. I think they're around $400-$500 plus the labor of installing it. The $100 units are usually mini spools which locks both axles full time. Good for drag racing, but not for street use.
2005 E350 Chateau - V10 - Agile Offroad 4x4
2012 CTS-V Wagon - For the baby...