Yeah, it's a bit confusing...but let me try and explain it.
BOTH systems (V8 and V6 4wd models) are the same because BOTH use the same center differential (TORSEN). The ONLY difference is that, in the V6 4wd models, there is an ADDITIONAL OPTION of 2wd mode. Both systems have 4-LO range.
The V6 4wd models can ALSO run in 4wd on ANY surface, just like it's V8 brother.
Both systems can lock and unlock the center differential. Both are TORSEN cente diff...thus, they are same. Once the center diff is locked, you cannot drive on dry pavement because of binding. Thus, locking the center diff should be done ONLY on slippery surfaces (mud, sand, off-roading, etc.).
The V6 4wd system is NOT the classic part-time system found on Jeep Cherokee or Wrangler. Unlike those, the Toyota system has a center differential (Torsen on '03 models) that can be lock and unlock.
If you do not lock the center diff, then there is NO binding on dry pavement when in 4wd (on V6 models)...thus, you can drive anywhere without any harm. Remember, on V8 4wd models, there is no 2wd option...it is always in 4wd. V6 models just gives it an additional option to run in 2wd.
Most V6 owners put their 4runner in 2wd mode on dry pavement...to save fuel. And when road conditions become more slippery or unpredictable, they switch to 4wd. HOWEVER, again, these same owners can just leave their 4runners in 4wd all the time.
VSC (stability control) and ATRAC (4-wheel traction control) are excellent for the conditions you described.
Hope this helps.