You're gonna have to be fairly specific in what happens other than the lockers don't work. When you put it in 4 low, there is a 4wd light on the dash that is supposed to illuminate. If that doesn't illuminate, then the locker switch won't light up indicating that the lockers are able to be engaged. The PCM gets the signal from the variable resistance switch on the t-case when in the correct mode, and then sends that to the dash light which then powers up the locker switch.
The most common failure is the t-case switch, second most common is the PCM has been replaced and it has the wrong software program in it. See if the PCM has a reman sticker on it and check the t-case switch.
You need to find out if it is 1" or 2" on the BL. 1" is good, 2" is bad. If you got it into 4 low, then it knows it was in 4wd.