The difference between AWD and 4x4 is AWD has a limited slip differential between the front and rear axles and 4x4 locks the front and rear axles together.
When you go around a turn on dry pavement (a high traction surface), all 4 wheels take a different path around the turn and therefore spin at different rates. The differentials that are on your front and rear axles allow the right and left wheels on each axle to travel at different rates, but when the front and rear axles are locked together (4x4), one front and one rear wheel is forced to spin at the same rate. This causes driveline windup and will break either axles, driveshafts, gears, etc, or will skid the tires causing excessive wear. AWD solves this by allowing a limited slip between the 2 axles so they can spin at different rates. This still allows all 4 wheels to be under power as needed, but won't cause driveline windup. In a limited slip setup, more of the power goes to the faster spinning wheel (the outside wheel in a turn, or the low traction wheel on a slipery surface), but not all the power like in an open differential system. Limited slip is typically good up to about a 60-40 split in power when one wheel is required to spin faster.
So to answer your question, AWD is a full time system that lets you give power to all 4 wheels as needed, but has a certain amount of slip between axles, and can only do 60-40 at best (usually no worse than 70-30 though) in low traction conditions and 4lock is a 4x4 that gives you that extra bit of traction when needed by locking the axles and giving you a 50-50 split.
I hope that isn't confusing without diagrams to show whats going on.
Last edited by 95_318SLT; 04-17-2009 at 12:20 AM.