You're forgetting about 4th gen 4runners. They have the following options:
2WD
4Hi Unlocked with Torsen limited slip center (like AWD)
4Hi with locked center
4Lo with unlocked center
4Lo with locked center
The V8 models are similar, but they have no 2WD option.
Good call! I've grabbed these descriptions from the '03 4Runner Manual (and edited out the shifter/display info that no longer applies). It does say "less wear" in 2WD, but less wear of what? And how much? For me, it reinforces the idea that wintertime driving with the center diff unlocked is just fine in all but extreme conditions.
“H2” (high speed position, two–wheel drive, center differential unlocked):
• Use this for normal driving on dry hard–surfaced roads. This position gives greater economy, quietest ride, least wear and better vehicle control.
“H4” (high speed position, four–wheel drive, center differential unlocked):
• Use this for normal driving on all types of roads, from dry hard–surfaced roads to wet, icy or snow–covered roads. This position provides greater traction than two–wheel drive.
“H4” (high speed position, four–wheel drive, center differential locked):
• Use this for greater traction when you experience a loss of power, such as wheel slipping, in the center differential unlock mode.
“L4” (low speed position, four–wheel drive, center differential unlocked):
• Use this for maximum power and traction. Use this for climbing or descending steep hills, off–road driving, and hard pulling in sand or mud.
• In this mode, the braking feeling that occurs when the wheels are negotiating a sharp corner is further reduced than in the “L4” (low speed position, four–wheel drive, center differential locked) mode.
“L4” (low speed position, four–wheel drive, center differential locked):
• Use this for maximum power and traction. Use this for hard pulling in situations the vehicle cannot negotiate even in the “L4” (low speed position, four–wheel drive, center differential unlocked) mode. Also, using this mode when driving down steep off–road inclines will help contribute to increased vehicle stability.