Thanks to some info on GoJeep's website, I found the answer.
To put pre97 bumpers on the 97+ and vise-versa, you'll need to change the bumper mounting brackets. The bracket attach to the bumper differently but they bolt up the the chassis in the exact same holes, both front and rear.
What I don't know is if the 97+ bumper end caps will attach properly to the pre97 bumper.