For the oil sender, I'd check the terminal connexion at the sender proper, I've seen vibrations loosen the connexion or break the wire through fatigue. Also, sometimes removing the sender, cleaning it, and putting it back in will help. Use never-seez or a very thin layer of RTV Copper when you put it back together - both are metal-bearing and will assist in grounding the sender.
For the voltmeter - I don't know where they tap the power in the ZJ, so you'll just have to trace that ckt out for yourself. I'd probably start with a good multimeter (analogue, for something like this) and a pair of piercing probes, and a drive. Have a helper watch the gage for you while you try to make the panel jump, and see if the multimeter jumps as well. If it does, it's a wiring fault. If it doesn't, the gage in the panel has failed (and it will probably be cheaper to get an entire panel from a salvage yard than to buy just the VM from the dealership...) Try to tap the voltmeter signal wire as close to the IP connexion as possible, to isolate the gage. If you have two meters and a set of long clip leads, I'd run one to the battery as well (but I don't know too many other people who have about five multimeters...)
If a vehicle has a voltmeter, it's a cinch that's it's not tapped into the best place available. My wife's 89 reads a consistent two volts lower than battery, and my 88 is all over the place. Both have a supplemental LED display that happens to be in a spare set of 12VDC power points I installed, and are viewable by the driver, which are more accurate than the gages. I'll rewire the gages properly later.
GOod luck - automotive electrics is the single most trying branch of mechanics. If you don't have a factory manual - GET ONE! Check with Leon Rosser Jeep - IIRC, they have most of them, and the prices are reasonable. Pay whatever they want - it will pay off greatly later.
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