Our Jeep CJ's can be full of electrical gremlins, and not from just what the P.O. did.
CJ gauges are electrically grounded to the Speedometer housing by their physical contact, the Speedometer which in turn is grounded to the steel dashboard by contact, then to the body tub. Now IF it exists, a grounding strap from the negative battery terminal to the body tub is how the factory and many have addressed grounding issues. Further steel is not the best conductor of electricty. Some, myself included have gone the extra step of adding a negative bus bar and copper wiring to address further grounding issues. Over the years I've found a couple of excellent write ups about gauges and electrical in general that I commend to your reading;