It's the centre diff that locks when moving to low range, or by pushing button on centre dash; forcing front and read axles to move at the same speed (or pushing 100% of the torque to both axles). The later years didn't have the CDL kick in when engaging low range, and it has been common to disable that silly feature also on the early years. Not all 100s have a rear locker, and none had a front one originally.
The 2 pinion front diff of the 98-2000 was weak, and likely to break, especially when reversing. The newer 4 pinion is sturdy enough.