Names Wyatt. I own an 89 Xj 4d.
 

Yesterday I picked up a 4.0 to replace the one that is in my Jeep. I looked at the casting numbers and it a 99/04. Is there any reason it won't work in my 89?

Welcome to the site Wyatt !

welcome to the forum...and the family.... You probably won't get much response to your question in the introductions section. If you post in the correct section you should get the response you're looking for.
At first glance.... can it be done? yes... how much work do you want to put into it... that's another story. Being of the Renix gen, it may not be completely straight forward, but not impossible.

Welcome to CF