Bill - OK, now we have a basic frame of reference.
ZJ (93-98)
WJ (99-04)
Both the ZJ's and especially the WJ's have stronger unibody frames then the (square body) XJ it sounds like you are considering replacing. Generally this probably wouldn't matter to you. However if you plan to off-road or (being in Michigan) the stronger unibody may fair better with rust etc.
I can't speak much on ZJ's beyond noting the 5.9 liter are popular due to the large V8. Perhaps others will jump with their experiences with ZJ's. Like all jeeps they tend to last a long time
Moving to either a ZJ or WJ does provide a few engine options (V8's) not available in the US XJ models.
The WJ's were a total redesign from the ground up. Much stronger unibody, better ride characteristics, creature comforts, etc. Of course this commands higher prices. The below WJ site may help you procure more information on WJ's.
WJJeeps.com - Jeep Grand Cherokee WJ 1999-2004
What general enhancements (if any) would you like if you were to replace your XJ (e.g. improved interior, stereo, heated mirrors, engine power, etc).
When I was researching my WJ purchase, the general consensus was the 03 and 04 models were the best in the WJ series. I went with 03 HO (i.e. Overland).
Hope this helps
Kent