FYI, if you put the shifter in 3 it will use gears 1 through 3. 1 and 2 if you put it in 2, and so on. Usually the automatic transmission does a good job, so you don't have to worry about it. The only times I downshift are to control my speed going down a hill, or if I really want to crawl over something in 4L. In 4L it tends to go to 2 really quickly unless you put it in L.
I don't think VSC was really explained in this thread yet. VSC mainly brakes individual wheels to try to keep you going in the direction you point the steering wheel. It might also reduce power to keep you from spinning the wheels and spinning out. It's basically designed to keep the handling as predictable as possible when you exceed the limits of traction. I find it to be useful on mud and snow, but it can get in the way sometimes. A lot of people here don't like it, and it can make driving off road less fun (but arguably more safe)
TRAC (or ATRAC or whatever) basically tries to get you moving/accelerating when one or more wheels have less traction, and it's a substitute for limited slip differentials. It might reduce engine power, too, so you don't overpower it, but it might not when VSC is off. VSC and TRAC might both run at the same time.
There are many other functions of the brake control system. ABS keeps the wheels from locking up to help you maintain steering during hard braking, and to maximize braking power on unevenly slippery surfaces. EBD balances braking force between the front and rear wheels (using the same interrupting mechanism as ABS, I think). Brake Assist boosts the brakes when it thinks you are making an emergency stop, but I find it annoying because after a hard stop if I let off the brakes, then brake a little more it goes back to emergency stop mode even if I'm just pressing the pedal lightly now. HAC tries to keep you from rolling backwards on hills, but it doesn't work very well. It helps to press the brakes hard before releasing the brake and moving your foot to the gas if you want to use this feature. There's no button to activate it, just roll backwards in Drive. DAC pulses the brakes to keep your speed between 2-4mph in 4L to help you maintain control in very slippery situations. It's not too useful most of the time, but occasionally it's nice. Auto LSD is I think only found on 2WD models, and it works like ATRAC, I think. I don't know why it's there, except that LSD is a selling point for 2WD trucks, so they didn't want to be left out.