Did a straight front to back tire rotation on the 96' XJ and the first time I'm up to 55 I get a fairly serious wobble that judging by it's frequency is almost definitely coming from one of the wheels. Is there any other possible explanation for this other than that one of the wheels is probably not centered exactly square on the lugs?

 

Maybe a broken belt that is more noticeable on the front.
Jack it up ans spin 'em. You should be able to see something.

 

If you only have this symptom after rotating tires, then the problem is one of the wheels or tires. Any good shop that deals in tires should be able to snake this out for you.

 

are the tires wearing evenly?

any balancing weights missing?

 

Going from quite recent personal experience, check to make sure the lug nuts are properly torqued, wheel spacers too if you have them. I rotated mine not too long ago and forgot to torque my spacer nuts....One was almost completely off by the time I decided to check them. It started to shake at 45 to 50 and if I didn't get out of the range, it'd turn into the death wobble quite soon. Not questioning your mechanical skill, but it could've been looked over.....

 

My jeep used to shake at like 56-57 then I rotates my tires and now it's like 60-61

 

Did you torque the lug nuts twice,Once in the air and again on the ground?

 

All tires are wearing evenly and no weights seem to be missing.

All lug nuts torqued before and after lowering tire to the ground using a 24" cheater.

For 4x4s I've read to do a straight front to back swap but someone else told me to x the fronts to the rear and bring the rears straight forward. Which is correct for 4x4s?

 

Rears straight forward. Then drivers side front to passenger side rear and passenger side front to drivers side rear.

 

Move all 4 of them around as needed to even out wear patterns. Especially cupping.