I have a 1996 XJ, bone stock. I recently replaced the rear U Joints as I had one fail (it sheared and the driveshaft fell down. pulled it out and drove her home in 4hi). About two days after doing this repair (and four or so starts) I noticed that upon start-up a quick rattle is evident, followed by a steady rattle in Park only. Upon switching to neutral or into gear, the rattle instantly disappears.

Any ideas on what this could be?

 

Flex plate bolts loose or cracked flexplate...assuming you have an automatic.

https://www.cherokeeforum.com/f51/ho...ocking-125547/
https://www.cherokeeforum.com/f2/pro...explate-46979/
https://www.cherokeeforum.com/f2/fle...roblems-32177/

 

Yup.

 

maybe the exhaust is tapping the crossmember. Sometimes when there is a load on the driveline(drive or reverse) the exhaust will lift off the crossmember and you will not hear it. It's real easy to check. Just get under there and make sure there is at least 1" of gap between the top of the crossmember and the bottom of the exhaust pipe.
If it's not this it's probably what they said^

 

I have to go with a flex plate problem, but hell what do I know.

 

Check both. All it takes is 15 minutes of noggin pokin.