Not all ham radios are created equal. Confusing, eh?
VHF/UHF radios like the one I have in my Jeep are great for short and medium range use. They are comparatively light, inexpensive, small, and easy to use. You can use them to talk to others in the same general area, or you can use a repeater to relay your signal to someone who is further away from you.
HF radios are the ones that can literally "reach around the world". They tend to be more expensive, larger, heavier, pull more power, and harder to use. You certainly can set up your Jeep for HF radios, but it will probably involve quite a bit more expense than a "regular" VHF ham radio would. The other problem is that HF radio starts to require larger, more complex, and sometimes more fragile antenna setups.
There's way more information to learn on this that I can reasonably convey in one post. There's a good bunch of it available on the net. Here's a good place to start
http://www.k0bg.com/He's writing from the point of view of operating on HF while driving down the street in a vehicle, but it's a good place to start reading because he covers many of the issues that present themselves when you start looking at operating HF out of a vehicle.