1990 Renix ECU's Twins Or? Or Not?
 

https://cimg6.ibsrv.net/gimg/www.che...2a9ef26ed1.jpg

What is the difference if any besides the part numbers?
The one on the left is from a 1990 Xj 4.0 4 wheel drive automatic trans. The one on the right is from a 1990 Xj 4.0 2 wheel drive automatic trans..

https://cimg9.ibsrv.net/gimg/www.che...7572b7f6ce.jpg
The one on the left is the 1990 jeep Xj 4.0 2 wheel drive automatic. One on the right is the 1990 jeep Xj 4.0 4 wheel drive automatic.
Only difference I see is in the upper left corners.
The one on the left has 3 brown blob looking capacitors where as the one on the right has 2 and 1 black cylinder shaped capacitor with a shinny top on it.
And some of the components are a different color.
All the pins are the same exact location and number for both plugs.

Would it hurt to plug in the 2 wheel drive ECU and try it? I'm thinking the one on the right for my 4x4 is bad but I don't see any bad solder connections or burn marks on the back side or any burned looking components.

IDK man. There appears to be differences in the circuitry. Very subtle differences. The power supply section seems to be different for one. That's the part with the aluminum heat sink around it. That may mean nothing.

You'll probably have to wait until Cruiser sees it.

The last letter of the part number is usually just a rev number. The computers for the same year should all be the same, regardless of 2wd or 4wd. For older Renix Jeeps, I don't think there is a difference between auto/manual either. Later models the auto ecm will through a code if it doesn't see the tcm. Manual ecm into a auto shouldn't complain.

They seem to cross over:

https://www.moparpartsoverstock.com/.../JR007842.html

The Mopar Reman catalog implies there is a difference between Auto and Manual (I think it has something to do with idling) but again the crossover of what you got there is noted:

https://cimg4.ibsrv.net/gimg/www.che...78e7a88cc2.jpg
They call the 6078 a 5300 instead of a 5600 but I'm thinking that's a misprint.

If you're using it for diagnosis purposes, just plug it in.