This is all I can find
(MX = a 4.0L 8.7:1 compression ratio engine with multi-point fuel injection system).
The HX is coming up as this: HX = a 2.5 liter = [150 CID] 9.1:1 compression ration engine with a multi port fuel injection system.
The YJ got the 258ci engine through 1990, then it finally went to the HO (ChryCo controls, OBD-I) 242ci. It was the last vehicle to be produced with the 258ci AMC engine.
89YJ 2.5L TBI, AX-5, NP231 SYE, Ford 8.8, Yukon Grizzly, Tom Woods Shaft, 33" BFG T/As, 4" lift, Warn VR10000s.
AF Jeep Club Tail #184