Sounds like you have a short from the switched ground to the frame - on the XJ, all the interior lights are constant powered, and grounded through door jamb switches. The switched ground wire you want to check is a yellow wire on every year I am aware of. What you can do is unplug it in various places where there is a connector and use a multimeter to measure voltage and resistance - measure voltage first, if it shows around 12V don't measure resistance as that is a lead going further toward the lamps. I believe on yours there will be a connector under the passenger side kick panel (right below the end of the dashboard) that will have a yellow wire, a pink wire, and a black wire plus some other colors. Disconnect and measure yellow wire voltage+resistance here, one of the wires goes towards the door jamb switches while the other goes towards the lamps. You should see about 12V on the yellow wire in the connector on the cable going up into the dashboard to the A-pillar and if I am correct, a weak ground (a few ohms more than the meter probes show when shorted together) on the yellow wire on the other connector. You can trace further from there.
Also, if you unplug this connector and your dome lights remain on, that means the short is further up the harness from the connector to the lamps. Luckily this harness is about 6 feet long and can be replaced in an hour or two with basic hand tools. It is also VERY common in junkyards in both RKE-enabled and dome-light-only configurations; I believe anything from around a 91 to a 96 will interchange with yours. If you have an OHC you may be better off patching the insulation after you find the place where it is worn.
