SR5 VS LTD

The difference is that the SR5 is normally operating as RWD until you put it into 4x4. Which locks the front and rear driveshafts together. The Limited LTD has a locking center diff, meaning that normal operation is AWD (like a Subaru) where both the front and rear are powered. 

The 5th Gen Limited does indeed have a full-time 4WD, often called AWD system because the torque split is variable. The standard split is 40/60 front/rear, and can be commanded up to 60/40. Its a mechanical 4WD using a Torsen 3rd generation center diff which is why the torque split is what it is (Torsen actually lists the T-3 as a 35/65 or 65/35 split). Other AWD systems can decouple the second pair of drive wheels and favor a more traditional FWD or RWD until traction loss is detected; not so with the 4runner.