Originally Posted by HkdOnJP
I didn't do the work myself but paid a shop to do it for me...
in 1995 they swapped in a 91 XJ 6cyl in place of my 89 YJs 2.5. I already had the ax-15 so they just bolted the 4.0 to it... some of the things they changed if I remember was the motor mounts, fuel pump, computer, exhaust and custom wiring harness and bellhousing (can't rememeber for sure about the bellhousing... i don't think they reused the 2.5 bellhousing). jeep ran awesome for about 10 yrs after that but near the end was having starting issues from the crankshaft position sensor or something like that. A common problem I guess with the XJs motors but easily replaced. I sold it just a year or two ago.
In other words, I was happy with the outcome. Sorry I can't contribute any technical knowledge about it though.