Molex,
In what is coined a "Part time" 4 wheel drive system which 4x4 trucks (tundra included) have, there is a differential on the front axle, a differential on the rear axle, and a transfer case.
When you are in 2HI, the only the two back tires get the engines torque (or power), where the 2 front wheels are just spinning along for the ride essentially. Now when you turn, think about how the wheels travel. For instance you are turning right. Ignore the front wheels in 2 HI, they are spinning freely, the 2 back tires travel different pathes and one is going to need to spin more times than the other to complete the turn. The differentials job in this instance is to detect that the outside wheel on the turn needs more engine power to spin it more, and send the torque that way to complete the turn. In a sense both rear tires get 50/50 of the engines torque IF you are travelling in an absolute straight line, and if you turn, they change that ratio so the outer wheel in the turn gets more of a share and the inside less of a share to manage the need to spin differently.
Now as soon as you turn the transfer case from 2HI to 4 HI or 4LO, the engine's torque is now sent 50 percent to the front differential (wheels), and 50 percent to the rear differential (wheels). If you were travelling in an absolute straight line, each wheel would get 25 percent of the engines torque evenly as distributed by their differentials to maintain the straight path. No matter what though, the 50/50 to the front/rear can't be compromised, it will always be 50/50. So the time comes when you have to make a turn in 4 wheel drive. Since the front and rear differentials do NOT talk to each other, and the 2 front wheels and 2 rear wheels travel in different pathes and require different numbers of spins than each other to make the turn, you in effect have a "conflict of interest" between the 2 differentials, 2 axles, and the 2 wheels found on each. In this case, 1 axle (and the 2 sets of wheels found on it) has to make a compromise, and it is usually the axle's wheels that have to spin more times (usually the front) that have to "slip" a bit to make up for the other axles wheels which require less spinning to make the turn. If you are on a slushy or muddy road, or somewhere where the wheels can slip to compromise, this is fine (and why you should only use part time 4wheel drive off road or in poor road conditions). If you are on dry pavement, you will put extreme pressure on your axle and wheel components because the wheels can not slip. If you do try and turn on dry pavement in 4 wheel drive, you will feel the two front wheels "jump," from extreme pressure put on them from fighting their own friction with the pavement as well as fighting the rear wheels and their less RPM spin rates in the turn. You will also need to make wider turns. This is commonly refered to as "understeer."
Here is a picture/link for more information.
http://www.4x4abc.com/4WD101/def_turn.html