Most part time 4x4 vehicles have a transfer case that locks the front and rear wheel speeds when engaged. This is why it's not good to run on the street in 4x4 as it will cause binding. The advantage being that most transfer cases are two speed. Full time 4x4/AWD vehicles have a center differential that can allow the front and rear tires to turn at different speeds, much like a regular front or rear differential allowing the outside tire to turn faster than the inside tire around turns. Some vehicles, like Humvees, have a transfer case and center diff allowing for full time 4x4 and a low range. Other vehicles like the Mercedes G-Wagen get around this by having an extra low first gear and a locking center diff instead of the transfer case. My truck has both an extra low first and a two speed transfer case. Some older 4x4 vehicles have single speed transfer cases which is really more like an on-off switch for the front wheels.center diff verses transfer case ?
Could you explain what you are saying?