Is my locking differential probably destroyed?

I have a 1997 Chevrolet S-10 Blazer with 4 wheel drive and a locking rear differential. I generally test it once a year (one wheel on slippery surface the other on good traction). It always worked well until last test 1 year ago; at that time it didn't want to unlock/release when I tried turning a corner . About halfway through the turn I felt it release and all seemed fine until this years test,it will not lock up at all. I am going to change the differential lube including the additive recommended for limited slip differentials, I just realized I was supposed to be changed every 30,000 miles and it now has over 112,000 and never been changed. I hope this will maybe free a sticking clutch. What do you think? Am i just being optimistic?