Quick question...Will an alternator from a Grand Cherokee with a 4.0L fit a Cherokee with a 4.0L if they are the same years? Thanks

 

Yes they will. Even the V8 Grands alternators work. I'm running one in my Jeep with no modification done.

 

1991-up, yes. Both use the Nippondenso with the regulator as part of the PCM.

Swapping the Denso into the 1990 and earlier XJ won't be a straight swap - the earlier rigs used either the Delco CS121, CS130, or 12SI - all of which are internally regulated (gas engines. The Diesel used a Paris-Rhone of some variety, I don't recall the details.)

Bear in mind that if your replacement alternator is rated for more than 117/120A, it will likely have a larger frame size - you'll need to grind the mounting area down about 1/4" for fitment of the frame. The wiring may or may not be the same, I've posted donor lists for swaps (if they're the same year, they'll probably use the same field coil connection. Just know that it may be different.)

The V8 ZJ/WJ/Dakota/Durango alternator is a fairly common swap into the XJ, so it's not like it's a huge issue. I've posted donor lists for securing an alternator "larger" than your XJ on several boards - including, I believe, this one. I need to work up a final version for my site, I just haven't had a chance to do that yet...