OBD 1 Start date in XJ models?
 

What was the first year model that Jeep XJ had the new OBD1 system factory installed? Thank you.

Not 1990, that I know.

1991. In 1996 Jeep transitioned to OBD2. The1996 XJ has a bit of both in it. 1987-1990 were the Renix years,

It started in 1991 model year (all of this info will be model year specific unless otherwise indicated). 91 and 92 have the round 3 pin CKP plug, as well as a 2 pin reed switch type VSS. They also have the fuel pump ballast resistor and its accompanying relay.
As I recall 93 got the newer CKP plug and moved to a 3 pin Hall effect VSS but retained the fuel pump ballast resistor and relay.
94 lost the fuel pump ballast and resistor.
95 may have changed the cruise control switch interfacing to the ECU a bit but I don't think it did.
Allegedly late 95 has OBD2 support but I certainly have never seen proof of this, I suspect it is people confusing model years and build dates and they actually have a 96 built in late 95.
96 was first year of the OBD2 system and JTEC ECU (aluminum 3 plug case instead of plastic 1 plug case) and changed many things such as the cruise control switch interface, diagnostic wiring, added a second sensor strip within the fuel sending unit that is used by the ECU to determine when to run the evaporative emissions control monitor program, etc etc.