AX15 4th & 5th gear rumbles

In 4th & 5th gear at 55 mph and up my XJ has serious vibration. 91 4.0, recent new NAPA hubs 200lb torched, drive shafts balanced, CVs checked, and tires balanced.

I don't see the relationship between vibration in both 4th and 5th gear within the tranny with everything balanced. It's getting worse.

Could the vibrations be from the 231 j tc? Has anyone else had this problem

And it doesn't rumble with the front drive shaft off.

I'd say that points to the front shaft as the possible problem. When mine crapped out, the end of the weld yoke(the part that goes into the centering yoke) was scored and started vibrating at about 60 mph. Looking at the cv without taking it apart was futile. Same symptoms as yours.

Last week I had the CV rebuilt and the shaft balanced with no change in vibes. Could a loose TC chain be the problem?

Maybe the front output shaft bearings in the TC? If you have the front driveshaft off, see if you can get any play by wiggling the front yoke. Any play is likely the bearings.

To check the chain, you can remove the fill cap and stick your finger in there to feel the slack in the chain.