Jeep had to do something for their 4.0s getting choked down by emissions laws, it seems like power output was in a steady decline from 96 to 98 (although the specs don't readily show it). The manifold first appeared on 99 WJs, which coincided with the vehicle redesign. I think the engineers had to do something about the lack of power with the 4.0 when compared to the new 4.7L V8.
As far as installing it, I have one on my 96 XJ and I could tell a big difference in power (my ass-ometer hit the redline). My setup has the K+N FIPK, bored out throttle body, updated intake and a Flowmaster muffler. It was pretty much a bolt in replacement except for extending a vacuum line and a sensor lead, everything else just bolted up. Your 97 should be the same as mine.
The only drawback that I have had is I lost a little bit of low RPM torque, but it really pulls like hell in the mid RPM ranges(1500-2500).
I got my manifold brand new from the dealer as they are hard to come by in my area, list price is over $200(I work at a Jeep dealer so I paid $170w/tax for the manifold, gasket, and replacement injector o-rings with the employee discount) but it can be bought used for much, much less if you can find one.
Well worth the time, effort, and money for me.
Bryan