Semantics, what does "4wd" actually mean?
A 2wd truck with open diffs is actually a 1wd truck; the second one wheel in the rear loses traction, you won't go any further. Both wheels do get the same amount of torque when traveling in a straight line and when both wheels have traction, but if either wheel loses traction it's game over, all the torque goes to the spinning wheel and the wheel with traction won't turn or help you out. So this is the same effect as if you actually had a single drive wheel which then lost traction, you're not moving.
A 2wd truck with a locker/limited slip, is a true 2wd truck. When the locker is engaged, you have 2 wheels which can provide movement as long as they have traction.
A 4wd truck with open diffs is actually a 2wd truck; if your left side of the truck hits a ditch and you lose traction on that side, you won't go any further; both the front left and rear left spin, the torque is "lost" to those spinning wheels and you won't go anywhere.
A 4wd truck with a center diff lock and with a locking/limited slip in the rear, is a 3wd truck; now when you hit the ditch on your left side, your rear wheels are locked so the right wheel will still get torque and provide you with movement as long as it has traction.
A 4wd truck with a center diff lock and front/rear lockers is a true 4wd; all diffs are locked, all 4 wheels turn at exactly the same speed, you only need traction at any single corner to get you moving again.