Time for you to crawl around and get dirty my friend.
Go here:
http://www.yuccaman.com/jeep/gear1.html and take a good look at the axle pics.
Get under you jeep and see which one you have. You probably have the Dans 35. There should be a metal tag bolted under 2 bolts on the cover.(It might be missing)
If it is there, giving that series of number to your local jeep parts dept. will tell you exactly what is inside. If the tag's gone, or you feel frisky, pull the dif. cover and look inside. Look where the end of the axle shows inside the side gears. if there is a half moon shaped clip on a groove on the end of the axle, it's a C-clip design. If there are springs between the side gears, it's a factory limited slip.
The "drop-in" or "Lunch box" lockers install in place of the spider gears and work well with tires up to around 30-31"
A "Full case" locker like the detroit locker can be installed at home if you know what the backlash is supposed to feel like, or if you can get someone who does it a lot to help. The gears don't need to be "set up", as the pinion never comes out. You just need to shim the carrier bearings for the correct lash and pre-load.
The "full case" or "drop-in" lockers work best off road.
I notice you're in Kansas. IF this is your daily driver and you spend a lot of time on pavement, your friend is right. The Detroit Truetrac is a great limited slip and won't cause the problems you get in snow/ice on pavement with a locker. It is a bit pricy, they cost almost as much as a full case locker.
To get an idea what is available try these companies:
Quadratec
4wheel parts wholesale
Currie
Richmond gear
After you get an idea what you want, talk to your friends and call around parts stores in your local area. A local shop may be able to get better pricing then what you find on the web.