Your '99 oil pressure sensor grounds through the PCM. It's a typical sensor used on your '99 XJ, in that it has a 5 VDC reference voltage supplied by the PCM, a sensor signal to the PCM, and a sensor ground to the PCM.
This sensor ground circuit is spliced into the ground circuit for the TPS, IAT, ECT, CPS, MAP, and others - all terminating inside the PCM.
If you are having oil pressure problems I would suggest checking the oil pressure using a mechanic's diagnostic direct reading oil pressure gauge. These can be rented (money back) from O'reilly and Advance auto parts.