It's often really hard to shift into low range if it's seldom used. You need to be at a full stop, tranny in neutral or park. Shift the t-case into neutral and then 4Low. You may have to work the shifter a few times, or move the truck, as the shift is not synchronized. Don't force it too hard, just firmly. If that still won't work, try lubing the shift linkage, and you may have to help it get into 4Lo by moving the linkage itself.
If and when you do get into 4Low, I don't think it will help keep you from getting stuck so often. That may be due to an open rear axle, tire slippage or driving habits. Low range is great for pulling a Dodge out of the mud, or creeping along really slow.