Just because Haynes makes this suggestion does not mean that it is necessarily a good one. The factory/DC catalytic converters on GC's (at least on 93-98's, not sure about 99 and newer) are prone to go bad. They are not quality units.
My experience has confirmed this. I have a 93' GC with the 4.0L I6. My first cat (factory) started going in late 96'- finally replaced in early 98'. My second cat (from dealer, installed by third party) started going bad in late 99' - replaced early 01'. This time I went to Mieneke - no problems since, but only 1 year on the cat. Mieneke has a good warranty - 5 years if I remember correctly. My next move is to get an aftermarket cat (CarSound) and another new O2 sensor (Bosch) when this one goes. Just taking the oppurtunity to do a little upgrading.
If you are not having problems with the cat do not change it out but do verify that it is the correct one for your GC.
93 ZJ 4.0, Rancho 9000's, Rancho Stabilizer, Splitfire plugs, BFG A/T 235/75/15's
More to come...