I use CB extensively during 4WD activities, and it works well in this application if you have a decent quality transceiver and a good antenna. Some of the handheld units from Midland, Cobra and even Radio Shack work well when paired with a good quality, vehicle-mounted antenna. But this is over distances of around a mile or two in the back country. In the same situation with the standard rubber-ducky antenna, we often have problems talking to other vehicles less than 1/2 mile away. At that range, you might as well walk to the rangers if you want to talk to them.
Also, as others have pointed out, unless you have specific information from the rangers in the area you'll be traveling, I doubt they monitor CB any longer. And even if they do, the chances that your signal would get through strong enough to get noticed is highly doubtful. There are no legal CB radio repeaters (not permitted under FCC rules).
Again, as others have mentioned, both SPOT and PLB (personal locator beacons) are the recommended gear for back country emergency signaling. They rely on built-in GPS receivers and satellite transmitters to notify SAR personnel of your emergency. PLB's also have UHF positioning beacons as a back-up to satellite transmitters. A satellite phone might be another option, giving you two-way communications capabilities, but a PLB or SPOT would be a more affordable and reliable choice if you're just worried about emergencies.