According to the '87-'90 parts catalog the sending units have different Part Numbers between the 2.5L & 4.0L. I looked them up at http://www.chryslerpartsdirect.com/ and they are listed separately (not to say they actually have any).
Do you have a picture of your old one I can compare to the ones I have? I have two here that I got from an '86 & '87 (?? I think, can't remember) at the JY, however, I don't really know if they work properly. Maybe we can trade the 2.5 for the 4.0?