I'm getting what I would consider poor gas mileage, even by XJ standards. I have a 20 gallon tank on my 99 Cherokee. I can fill up and the gauge will read below "E" right around 200 miles. That's like 10 mpg!

My rig is bone stock and I keep it mostly empty. I've also done some things to try and get better economy from it - like neon injectors, plugs, wires, and distributor tune up, APN header, larger OD exhaust from the downpipe back, new cat and muffler, new after cat O2...

I'm running out of things to check for here. The last thing I can think of is that my fuel pressure drops after I shut the vehicle off. I have to do the poor man's prime to start it up if it sits for more than 15 minutes. I don't think an injector is leaking because I just replaced them, and this problem was happening before I did that swap.

Suggestions? I'm tired of hearing about peeps rocking 33's that get better mileage.

That's not the correct way to check mileage. When you fill-up pay attention to how full it is then write down the starting mileage. Try to return to the same station to refuel and watch how much you get in it to the same point as last time, same pump if possible. Then write mileage down again and divide to get the MPG, do this several times and you will have a much better idea.

^^ Try this first. My fuel empty light comes around 1/4" tank. When I fill up it takes about 15.5 gallons. If yours is the same, you'd be getting 13 MPG which isn't terrible for vehicle driven mostly in the city.

If after determining if your fuel economy is still low with the method Fred described, check for vacuum or air leaks - vacuum lines, manifold bolts, and pre-CAT O2. If the pre-CAT O2 senses extra oxygen from an air leak the engine will run rich and you will get dumpy fuel economy but you may not get any codes/CEL.

I'm a total data nerd. Embarassingly, I have a yr's record of every fill up for my stock 99. Milage, gas brand, note of where needle started and finished (always an interesting variance there even though I always remove nozzle at first click), date, and whether time frame was predominantly city or hwy. And I, too, prime before every start.
From this I find a very big city vs hwy difference-up to 10mpg diff. Winter produced a big mpg drop (frigid weather so I regularly warmed it up for 10 ir 15 mins prior to commute). I have been as low as 10.5 mpg in an all city driving period and as high as 25 mpg during a 70mph 500 mile roundtrip.
Moral of my unhelpful and embarrassing neurosis: I would chart condns for awhile to identify a true pattern before driving yourself more nutso haha

OCD! Lol. It's a brick, of course it gets bad mileage compared to their stuff. But a Hyundai Elantra won't haul camping gear on back country roads or push through snow in the winter. Just love it for the utility.

oh wow! lol. Yes, I've been somewhat charting my fill ups and have noticed some trends. I wonder if having to prime the rail before a cold start hurts. And if it's not the injectors leaking, then is it most certainly the fuel pump that needs replacing?

Well general advice is change the filter, then if you still have issues, do the pump. You do have to drop the tank to change the pump.

Just a for instance my 99 has a 4 1/2" lift rough tread 33x12:50x15 tires, roof rack large, winch/bumper, rear swing-out tire carrier bumper, heavy in the cargo area, armor, with 4.56 gears/Detroit Lockers. Combined city/highway driving 15.7 mpg. Before gear change (3.55's) was getting 9-10 mpg avg.