Fixed length control arms for a 6.5" lift on only a 4.5" lift would cause excessive caster, and would point the pinion angle down enough that I could see having some problems with vibrations at highway speed. That may be the problem in this case... check your caster angle with the rig sitting level by putting an angle finder on the top of one of the ball joints and seeing what it's at and ride height... and post some pics of your front driveline angle.
However, I don't think it's entirely appropriate to compromise caster for front driveline angle especially on a rig that's driven at highway speeds, that's not lifted to the sky. In my experience, there's plenty of "wiggle room" in how straight the front pinion angle is pointed at the T-case, and I always shoot for the factory 6-7 degrees of caster rather than try to point the driveline up. I've never had driveline vibrations from the front DS on any XJ lifted up to 6", including an XJ with 5.5" coils and a LOW PINION 44. The angle on the front does not have to be perfectly pointed at the front t-case output just because it's a CV driveline.
As far as whether you have wear in the slip... it's perfectly possible for the slip to wear out. If there's excessive play in it, it should be replaced. They usually have that blue plastic stuff coated on the slip so that it's a nice, tight fit. I doubt this is your problem though, I'm going to guess it's due to the fixed length CA's that are for a lift height higher than what you have, which is changing the caster to something more than 7 degrees and pointing the pinion angle down TOO far.
