Well, here's some stuff I've come across.
Small cans are good for getting small fires. Stuff like smoldering wires under the dash that haven't flamed. For that sort of thing though you want the extinguishers close by not at the back of the Jeep. You can to get these ASAP before they are allowed to get big. But also keep in mind that some like Halon displaces air so you do not want to start using it while moving in an enclosed vehicle. I'd be wary of any of the aerosol style extinguishers though. They lack any type of gauge so you can't tell if they are in fact ready to go. I've heard of several instances where some cans were with little or no pressure.
I've also heard there is a worry with magnesium in cars. When it gets hot and is quickly cooled you get get an explosion.
So water and AFFF (foams) would be a problem. The magnesium attaches itself to the Oxygen and releases the Hydrogen which ignites. The transfer case on some NP/NVG are magnesium from what I've heard. Though the Jeep's should be aluminum. I've heard some BW's are still magnesium in early 2000's GM's.
http://www.youtube.com/v/qU6arBcC8nQIf you have a Dry Chem, be sure to shake it up once in a while. The dry bits can and do settle though I've never seem it happen to one in a car. They use sodium bicarbonate (NaHCO3, baking soda), potassium bicarbonate (KHCO3, nearly identical to baking soda), or monoammonium phosphate ((NH4)H2PO4). They all will make metal corrode or rust if not cleaned up.
I've heard that Halon or the new stuff. (Halon is a CFC) can damage engines if ingested. Then again if you have a fire, I think that's the least of your worries. I think in most cases it's going to be an electrical fire. No backfiring carbs to worry about with our EFI.
Some guy's seem to say you could carry a 20lbs fire extinguisher. I think that's a bit crazy but if you have the room why not.