As a general rule, for leaf-spring-suspended Jeeps, the higher the lift, the more problems you are likely to run into, such as steering, driveline angles, etc. So you will likely run into more of these problems with a 6" lift than with a 4.5" lift.

What size tires are you planning on running on your YJ?

1977 CJ-7, fiberglass body, AMC 360 w/ headers, DUI ignition, Edelbrock intake and Holley 4150 carb, TF999, Dana 300, 4.56 gears lockers, York air comp, 4" susp lift, 2" body lift, BFG 35" M-T tires, Megashifter, AGR pump & box, REP8000 winch.