WJ steering knuckles can be used on a ZJ. but it is not as simple as just using the knuckle itself. you must use WJ brake calipers/pads/rotors as well as ball joints, a spacer, bearing hubs i believe and possibly a few odds and ends. it is a good upgrade that a lot of ZJ owners with lifts or bigger tires run to improve stopping performance; take a look at the brake calipers side by side and you'll see how much bigger the WJ set up is than the ZJ.
5.9 ZJ... 4.5" Rubicon Express short arms. Bridgetone REVO 2's... 265/75/16.
2004 WJ... OME HD coils, Bilsteins shocks, JKS adj track bar, MOOG SS, ADDCO rear sway bar with ext., Bridgestone REVO 265/70/17.