Its more important you learn how the drive train works, thus understand what you shifting or switching too, then you'll figure out when its appropriate to shift to different modes in what conditions.
The different terms get misused all the time, arguably the Commander is AWD NOT 4WD, and technically 4X4 is definitely the wrong term. Regardless, and you can do the research on you own for other kinds of 4WD, this how the drivetrain on the Commander works.
The Commander did come in RWD versions, this does NOT apply to them and you definitely don't have one.
QTI, QTII and QDII, normally are always in 4WD Hi, or AWD, a.k.a. Full Time 4WD.
Other 4WD vehicles, like earlier Jeeps, you normally were in RWD with the front drivetrain disconnected, and you had to throw a lever to engage the front drivetrain and put it into 4WD Hi. That is what is confusing you, you think your 4WD must be like on these, it isn't. Unless you've thrown the lever to put it into 4LOW, you vehicle is in AWD, 4HI or Full Time 4WD all the time.
You have open differentials in the front, XFR Case and rear (QDII is limited slip). As you turn and go around curves, the tires will spin at different speeds, open differentials allow the wheels to spin at difference speeds with no resistance. QDII has limited slip differentials, that allows wheels to spin at different speeds, but with some resistance, that little bit of resistance to different wheels speeds helps keep the power to all wheels in slippery conditions. QTI and QTII uses traction control to the brakes to limit slip between wheels, so they have a virtual LSD, while QDII has a real LSD.
As well, in normal mode, 4HI, AWD, Full Time 4WD, you have electronic traction control, stability control and ABS, that will activate individual brakes as necessary to help maintain stability and traction. Again you have all this, simply by starting the engine and putting the transmission lever in drive.
If you press the button on the dash, all that disables is the traction control feature that allows wheels to spin, there is a dozen other features still working in the system that are NOT disabled. In deep mud or snow, you may need to spin the wheels to get moving, in most cases you do NOT want to spin wheels and traction control prevents it. So in some cases in snow or mud, the traction control is keeping you from spinning the wheels to get moving, and you think spinning the wheels will help you get moving, then press the button and turn that feature off. When you press the button and turn off the traction control, you get a yellow light in the dash that is a symbol looking like a vehicle skidding or fish tailing on the road, to tell you the traction control is off.
In the Owner's Manual (OM) there is description on how to turn off more feature by holding the button down, it will turn off the directional control, to allow you slip and slide around, fish tailing, if you think its necessary to slog through mud or snow.
In most conditions, the normal mode is all you need, and on the Commander you have it by just starting the motor and putting it in drive. In a few conditions you might want to turn off one or two of the traction control features by using that button on the dash. So, when you encounter snow, you don't have to do anything at all, well except adjust your speed and driving habits and be more careful, the Commanders 4WD is already working and will adjust on the fly as necessary. You do have the option to turn off 1 or 2 traction control features, if you think it is necessary to spin wheels to get moving in the snow, by pressing the button.
So, say you get into really bad conditions, really deep snow, mud, sand or really loose soil off road. QTI, you can't do anything, it doesn't have 4LOW. But QTII and QDII, you can put it in 4LOW. NOW this is important, instead of trying to remember what are the exact conditions you need 4LOW, you really need to remember how your drivetrain is going to act when your 4LOW. If you know how your drivetrain is working, you'll know that, Holy Crap this is a hard high traction surface it will tear up my drivetrain if I keep it in 4LOW, that is what is important.
So, 4LOW, when you pull that lever up, OM says you should be moving forward at 1-5mph while you do it, "I believe". The XFR Case will shift a gear set in it to lower the rpm coming out of the transmission. It will disable the wheel spin control and directional stability of the Traction/Stability Control, which light the yellow Traction Control Warning Lamp on the Dash. It will also lock up the differential in transfer case that allows front and rear axles to spin at different speeds. And since you have QDII, QDII will also lock the differentials front and rear.
Most important to know is the differentials locking. Regardless if its QTII or QDII, just the XFR case or all three differentials. Once a differential locks, all or some of your wheels spin at the same speed, there is no way for all of them to spin at different speeds. When you're driving a straight line, all the wheels spin at the same speed, unless one wheel is slipping in mud or snow to make it spin faster, so even on road going straight a differential isn't turning. Its when you turn, wheels want to turn at different speeds. With a locked differential, when you turn the wheels can't spin at different speeds, they will continue to spin at the same speed. This is good and bad.
With a lock differential, the only way to turn is for some of the wheels to slip on the surface. If its a very loose surface, like deep mud, deep snow, deep sand, it easy for the wheels to slip and spin at the same speed while you're turning. In fact, if you're going slow, that can help to move, slog through the deep stuff. BUT, if you're going fast and cornering on road in the rain, wheels slipping is very bad, you slide and even spin out. More importantly, on a hard surface or even a soft surface with lots of traction, that can be bad, if the tires can't slip when your turning, and they dig in and grab the surface, which they will on road dry, or light snow or rain, etc. the drive train will try to still spin the wheels even though they won't let go, and that can result in breaking the driveshafts or tearing up the drivetrain, premature wear on the drivetrain, etc.
So, before shifting in 4LOW and locking the differentials, just think to yourself. I'm going to lock the differentials, all the wheels will spin at the same speed, meaning they have to easily slip and spin over the surface. Is that what I want? Are the conditions such that the wheels can slip and spin over top the surface? If the surface won't let the wheels spin easy, I will be tearing up my tires, driveshafts, axles and XFR case.
Also going into 4LOW, engages that extra gear set to Underdrive the drivetrain. That means you go even slower per engine speed. That gives lots of torque at the wheels to climb over obstacles or up steed hills, but the vehicle can only crawl along at a slow speed. So, if you're driving along with the engine at Redline RPM and want to go faster, you've got to shift out of 4LOW. Read the OM how to do it properly, I do NOT think shifting out of 4LOW at 20mph and the engine at redline is the proper way to do it.
Last edited by Mongo; 12-30-2015 at 11:33 AM.