I'm averaging about 300km (less  than 200miles) to a tank of gas. I barely make it to, then back, from where I want to START wheeling. Has anyone come up with a good way to extend the range? I've considered making a swing away tire carrier with a tank holder, a carrier that fits in the receiver or just carrying jerry cans inside. None of these really appeal; they all have big drawbacks. Anyone tried something different or have any suggestions?

That's about right for a 2 door, I could get about 200 miles and I was looking for gas.

The best thing I thought up was a custom fuel tank with more capacity, but to get any
real increase, like 5 gallons, you would need to have a body lift.

I have an 89', and it came with a 10.2 gal tank, some of the later GEO Tracker's had a
12 gallon tank, not huge, but it represents a 20% increase in fuel capacity. You might
look into what years had the larger tank, also the 4 door tank does not fit a 2 door, too bad

Wild

I carry a 5 gal jerry can next to my spare on my swing away and don't even notice it even when it's full. Great insurance.

I started to make a swing away, got a latch off a Nissan SUV and made a strong hinge but decided that, as I could use the stock tire mount for my spare, I was adding a lot of weight and drag just for a fuel can. Still have the set up if it comes to that.
 Didn't know there was different tanks. According to the manual my 92 has a 11 Gal (42L) tank. It would only be worth changing if I had to replace the tank (only 27km further per tank). I'll look some more at it.
Forgot to mention I looked into making a roof rack but it also adds weight and drag, only it also raises the center of gravity and is even harder to make well than the swing away

 I have a 20 gallon steel boat tank that I use for long runs. I put in the back after flipping the rear seat up, take the fuel hose and run it over the taigate and into the fill pipe. Close the lid and run. Gravity feeds the main tank, and when the gas hand finally moves, I start to look for a gas station to refill. I can run about 450 miles this way BEFORE I even get into the actual Tracker tank supply. Then I get to where I am going and don't need the added capicity, I simply remove it and stash it, usually in a friends trailer, or something similar. It's nice watching everybody else in the group have to pull in for gas, and I sit there and wait on them, or go on and run "recon" to find where the bears are hiding.