When turning, the outside wheel is essentially unlocked and is allowed to rotate faster than the diff. If you throttle it up enough, it'll engage and you'll have a 50/50 split between the wheels which will cause some extra tire wear on both side.
I've got a Detroit No spin in the rear end of my '86 Chevy and a TrueTrac in the front and have been very pleased with the performance. That said, I can definitely feel the locker in the rear while on the road, especially while making sharp turns. Sometimes it catches and keeps both tires tied together (seldom, but it happens), but usually it just creates a jump and a small chirp when the trans shifts during a turn. This could be because I've got a hopped up trans that shifts quite firm, but I'm not sure. My Chevy is a long bed single cab, so not quite as long as a crew cab Superduty, but it's close. Every once in a while, the rear end will bind up in reverse and jump and pop VERY loudly (to the point where I thought I blow it out the first time it happened). I've been told that this will happen from time to time, and I'm used to it now, but it still makes me cringe when it happens (twice a year).
In my Ford, I've got a TrueTrac in the rear and have been very happy with it. I no longer spin a single tire in the rain and have very much improved offroad performance. That said, I'm considering putting a locker in the front just to maximize everything and still have very nice road manners. I don't take this truck off road all that much, but Powerlifter, I really hate being stuck in a ditch in a big 4X4 truck. Not sure when I'll have extra $$ laying around for the front locker, but it's definitely on my list of things to do.