Yeah, You need a SYE to run the CV style driveshaft on you XJ. The stock setup is like many rear wheel drive vehicles...the rear driveshaft has a yoke that slips over the splined end of the transfer case output shaft. The older XJ's had an exterior seal that mated to the driveshaft...bad if you overflex and pull the driveshaft out b/c your fluid can leak out. The newer style incorporated a seal before the end of the output shaft so the driveshaft could be removed without all the fluid leaking out (see above pic).
My recommendations are:
If you have the newer XJ (97+) with the transfer case that is sealed and you are on a tight budget, then go with the hack 'n tap b/c it is cheaperr than a complete HD SYE (like advance adapters style). Although you will still need to get the adapter plate to use the front XJ driveshaft. It runs about $50 or so at NAPA.
If you have the pre-97 style, then I would suggest a complete HD SYE b/c it will be cheaper than than the hack n' tap for that style b/c the hack 'n tap is about the same price, plus you have to get the adapter plate, which puts it well beyond the price of the HD SYE.
I would go with the PORC (http://www.performanceoffroadcenter.com
) SYE if you are looking for the heavy duty setup. It comes with a new main shaft for the TC and the install isn't that bad, although it may sound scary on paper.
My other question would be if you are getting a true 6" lift, as in, it will sit at 6" when on vehicle, or are you getting a lift that is advertised for 6" of lift, which means it could sit higher, which most lift do. If you think your end lift will be 6", then you could even use the front driveshaft from an XJ (which has a CV joint) and save yourelf some money over having a custom one made.
I have 6" of lift on my XJ from my 4.5" RE superflex kit and I installed the the PORC SYE and went to the junk yard and got a front DS from a late model XJ for $75 (some guys get them cheaper than that depending on your area) and did the whole thing for about $270 (after buying some tools, fluids, and RTV sealer.