Weight / tank type / thievery
Rev, I'm not sure what makes me more mad about your post- the fact that you have the nerve to invade the EOC as yet another dirty X-Terra owner, or the fact that you're throwing its ground clearance and sure-footed offroad capability in our faces by showing that it's comfy enough over big rocks for a Bichon! ;-)
(Yeah, you caught me... I've spent a lot of time around that kind of dog and my ex-fiancee's X-Terra, and I miss them both.)
My interest was piqued as I started thinking about where the hell you'll be driving that you'll need more than a full tank of go-go juice between rally points...
I agree with the majority of posts. Unleaded gasoline, depending on specific gravity (some slightly vary based on octane level, winter/summer mix) is 6.3-6.5 lbs/gallon. Adding the weight of a fancy wheeled container brings the weight up to about 8lbs/gal, as I think someone mentioned above. Your best bet from most standpoints would be to carry several smaller jerrycan sized tanks, bungeed to the roof and run a cable bike lock through the handles.
If you're worried about managing the tank removal in rough terrain, or just having an accidental drop off the top of the E, you may consider a tank with a spout that you could have on all the time... You could mount the tank spout side down (presuming that it seals properly) and attach a length of vinyl or polyethelene hose to the spout, with a plastic ball valve close to the beginning of the hose. This way you could uncoil the hose, put it down the tank fill line, and just open the valve and let it fill itself.
Of course, I have a penchant for over-engineering everything. If I were going on a big badass expedition, I'd probably do something crazier. Probably something like...
Fabricate a hanger bracket that would mount via the attachment points for the gas tank skid guard and hang a long, skinny tank on the left side of the existing fuel cell. I imagine I'd use a fat piece of high density polyethelene piping with a small remotely-controlled pump feeding a line that goes directly in to the existing fill tube... ...but that's just me.
For what it's worth, just buy the cheapo cans. If you find a 2.5-5gal tank with a handle at the correct height, you even might be able to remove your rack crossbar, run it through the handles, and reattach it. Not the most convenient each time you do an auxiliary fill-up, but definitely the most secure. Have fun!