When I hear the word "locker", I picture a positively locked differential. Anything from a Detroit to a spool to a welded set of spider gears. Those would have no "differentiation" (is that a word lol). Those would bark the tires on pavement while turning. Those would help propell you forward though a mud hole or up a pile of rocks. Those would not let the tires/axle shafts change speeds from each other.
When I hear the words "selectable locker", I picture an ARB or an Auburn ECTED or an OX locker. All of those can be in an "open" position and function as an open differential, then when you push the button or pull the cable, BAM full locker, full power both axle shafts.
Now when I hear about the gov-lock, since it has clutches, since it lets the tires spin at different speeds, since you have to have wheel speed up to make it work (at least a worn out one), and especially since I've been stuck with a non functioning one more times than i would like to try to remember, I do not consider it a true locker. They are prone to failure when abused. Now for a street driven truck, it's just fine, but street driven trucks do not need lockers.
I found this: http://www.youtube.com/watch?v=6Zit49L-7_A
Maybe it's the push button lockers in a couple of our tractors or the off roading background I have or all the time spent on pirate.
Also when I bought my 91 k5, it had one in the back, not stock I'm pretty sure. It exploded on pavement and left me stranded. I was not abusing it at the time and the truck had 31x10.5s on it. Only thing salvageable from it was the pinion, the rest turned into shrapnel. I was not pleased.