The front bumpers are universal but you need to brackets that hold them as well. Just put a 95 front bumper (old style) on my 97 xj (new style), and the bumper bolts differently so you have to grab the bumper bracket as well. The 96 and older brackets bolt up perfectly to the 97+ unibody.

The 97+ end caps will work on your 97+ xj with the 96 or older bumpers, but you may have to drill some new holes to get it to bolt up to the bumper itself if you want to use all the pre-existing holes in the end caps. The side profile is the same though - it will definitely work and the end caps bolt up without any gaps.

The inner attachment points for the bottom of the end caps is the same spot that the bumper bolts onto the bracket - those holes line up perfectly. To be honest, it's a strong enough attachment point so it's really all you need, but if you want to use outer attachment points as well you have to drill new holes, either in the end caps or the bumper. The vacuum canister doesn't fit as neatly, but still fits. The older style canister was longer than the newer style, so you can only use 1 of the 2 holes unless you drill new holes in the bottom of the bumper. Might as well just mount that in the engine bay beside the coolant overflow, but you can still bumper mount it using only 1 of 2 holes, and it will be just fine.

The only thing I noticed, and this is just a minor thing, is the old style bumpers leave 1/2inch more room between the top of the bumper and the bottom of the front grill than the new style bumpers, but it doesn't look weird or out of place.

Source: literally juts did all this last night.