Not sure where you live, but GMRS radios don't require any licensing here in Canada and they're available at virtually every electronics store, cheap. The audio quality and portability are better than CB, but CB tends to have more range, pound for pound (proper installation and tuning, good antenna, etc).
There are radios that will do HAM/CB/GMRS/FRS, but they're spectacularly expensive and might require hacking that's technically illegal. Your best option is to have a conversation with your wheeling/exploring buddies and agree on a method to use, or keep a CB and GMRS in the truck if you're trying to hedge your bets in the wilderness. You can make a harness that allows a plug-out-plug-in switch between different kinds of radios, but as others have said, they all require different antenna lengths (and connectors, I believe), so it would be a pretty crappy task.
S