No, a 92 under dash harness will not work in a 91. The 92 harness is completely different than the 91. 1991 was the crossover year, first year for the 4.0L motor in the YJ. An 87 to 91 under dash harness will work in your 91 YJ.
As far as the engine harness' from 91-95 go, they are the same and are interchangeable. The only differences will be that some of the sensor connections changed between the years, but this is easily remedied. For instance, the '91 engine temp sensor for the gauge had a single wire push on connection, were as the '92 and later had a weather pak connector. Same is true for the oil pressure sensor, but the electrics is the same, only some of the connectors for the sensors may be different. If I missed something, I am sure it is easily overcome. The OBDI engine harness is essentially the same and is compatible from '91-'95 except for the few sensor connections that are easily overcome. Pre '91 is a different ball game altogether.
Thanks for the info. I'm only missing one section of the harness - the section from the firewall to the speed sensor on the tranny, so I suppose I better stick with a 91...
Good choice, the speedo drive for the '91 (outside of the engine bay wiring) is different from all other years. The '91 uses both a mechanical and VSS output sensor at the transfer case outlet. On the '91 this is a two wire output and has a reed actuated switch for the VSS and still has the mechanical cable drive for the speedometer. '92 and later does not have the mechanical cable output and supplies the speedometer and VSS info with a hall effect sensor to the ecm.