I did a 4.2 to 4.0 swap in my 87 YJ, but I got a 92Yj 4.0 with harness and computer. Still many wires to reroute.
Check out the build thread here--->Project YJEEP
This pic is before I finished the wiring:
Doing the '98 OBDII swap will involve much more than I had to do. The Grand is much different from the YJ in many respects. I'd recommend getting a 91-95 4.0 wiring harness!!!
Good luck with the project.