Anyone know what years the 3rd gens have a limited slip differential? Or if it was just an option?

 

Check out the below link.
IIRC the LSD was in the NM through '02 stock from the factory.
I'm not sure if Mitsu kept the LSD when they added the traction control to the later model NM's

Outer Limits 4x4 Board • View forum - Mitsubishi

Try looking it up in the Tech Database thread or the Tech Bible thread.
HTH