Big differences 87-90 to 91: engine management changed from Renix to OBDI, EGR system eliminated, HP increase, o2 sensor moved from header to downpipe.
And there lies the rub for the header: the Renix header has two bungs--one for the EGR tube and one for the o2 sensor, the 91 and up HO headers have no bungs, or nobungus for short.
So, what are your choices? Well, you can pay a lot of money for a "performance" header for a Renix--Pacesetter makes one--or you can purchase a 91+ HO header and install a couple of bungs.
I think the best header out there, not a performance item, is the APN:
http://www.ineedparts.com/index.php?target=products&product_id=15343 but if you buy one don't use the gasket that comes with it, buy a quality gasket locally.
As lawsoncl stated there never was a 4.0 non-fuel injected engine in the XJ from the factory.
You will get more "bang for your buck" with a stock header and a performance CAT-back exhaust system.