Just recently went through all this... :)
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You've got 3 possibilities:
a) Gauge
b) Sending Unit
c) Wires (inc. ground)
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Good news is you have a partial reading...
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Ground test: Hook up a jumper wire between the base of the sending unit to a good ground (I typically use one of the bolts in the passenger compartment). That should not change the reading at all. Second test is if you have VOM meter, hook it up between the base of the sending unit and a known good ground (or the negative terminal of the battery would be even better). Should read 0 ohms.
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Sending unit: this is likely the issue--if the sending unit is old, and especially if you've used fuel with ethanol, things are likely very rusty, and that rust may be preventing the arm of the sending unit from achieving full travel.?
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a) Peek down the filler neck with a flashlight--any rust visible?
b) Grab some copper electrical wire (12 or 10 ga if you have it). Bend it into a LONG "U" shape (the U shape will give you a bit more strength and the ability to "rotate" it). Bend that long contraption so it goes into the fuel filler neck until it gets to the float, and then see if you can manipulate the float (i.e. push it "up") and see if the reading changes...not that if things are really rusty you may not be able to move it.
c) Above tests don't require any disassembly - but (especially for a CJ) it's probably easiest to remove the sending unit from the tank (should be 5 small screws). Betting that if you pull it out it'll likely be pretty rusty. However once you have it out, see if the arm moves easily throughout its range. Also? you can hook it up (you need a ground) and then see if the fuel gauge moves appropriately. Note that even if the armature moves the full range, then "internals" may have rusted, and no longer be making contact. Additionally, you can hook up your VOM across the fuel gauge and measure the resistance as the armature moves across its range (it should constantly change, and there should be no "dead spots"). Note that the gauge generate very few ohms (so you'll likely use one of the lower ranges on your VOM). Wish I'd have written down the ranges when I purchased my new sending unit).
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Gauge: again, I'm betting the sending unit is the problem, but if all checks out ok, then you can test that gauge by hooking it up with a variable resistor in place of the sending unit, and seeing if you can get full range of motion from the gauge.
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"Common things occur commonly"
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Given the above adage, this is likely a rusty tank, and sending unit. (I had my Jeep in storage, with fresh gas, and when I got it out of storage the sending unit was completely rusted--an AMAZING amount of rust actually, so I'm betting on rust). If you pull the sending unit and it has any rust at all, I'd just get a new one (not worth the hassle for the price). HOWEVER, once you have the sending unit out get out your flashlight and take a REALLY GOOD look at the tank--you'll only be able to see some of it, but if you see rust, I'd take the opportunity to put in a new tank. Again, for a few dollars you will have peace of mind, and you won't have to deal with the other issues that a rusty tank can cause (mainly plugged lines, fuel pump issues, and plugged carb jets).?
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Hope this helps.
Sean Kerns