I have a 87 3.9 with the compression in 1 and 3 very weak. I was giving a 98 3.9 motor and tranny.Could someone tell me other than the computer what will have to be change?

 

Find a wiring harness out of a 92 to 95 dakota and a brain box. It may be somewhat complicated, I'm not familiar with the 87 wiring.

The 98 motor is the same as a 92 up, so for you you will need to find 92 to 95 stuff to make it run.