If you've eliminated the steering box spacer/bolts, you may want to take a second look at the upper and lower trac-bar mount. In your picture I can see that the upper bracket is a TNT one so it's not the bracket but it may be that the bolts that attach it have backed off a touch. On the diff side, check the bracket to see if it has a crack in it. One of our XJ's had a similar pop which was difficult to find because of where the diffs side bracket had cracked.
The factory inner steering box spacer is made of aluminum. If the bolts had been loose at one point, it can become damaged and make it difficult to get the bolts tight...check that too. But since removing it is a pain, leave that until all other diag has been done.
The bolt/bolt hole in your TNT unibody side trac-bar mount is punched to be true-sized to the bolt that goes through the small trac-bar bracket. There shouldn't be any slop here. You can check it to be sure but I'm fairly confident that you're good there.
The bolt hole in the diff side trac-bar bracket could have wobbled out if issues caused it to do with a previous trac-bar setup (worn, sloppy, etc). The trac-bar on this end has a factory style double bonded assembly (bushing) which has the factory sized through hole in the inner sleeve. The factory bolt (or same size new one) will be the correct bolt for that hole. If that's the bolt you have, you should be good there as long as you (or a previous owner) didn't drill that bracket to accept a 7/16" bolt at some point.
Also, just for fun, check your coil springs. If th retainers on the lower coil buckets are no longer there, the coil spring can be shifting ever so slightly. It's not likely that it does it while steering in the driveway but stranger things have happened.
Usually a balljoint clunk won't happen in the driveway with slight steering adjustments unless the balljoints are ham, ham, hammered. Worn out ones will give you most fits under power while going over bumps, cracks, ruts..etc.
Attempting to "feel" the clunk is a good idea. It will get you in the general area of the issue but remember that clunks can translate well through steel. When you find the clunk, it will usually be a smokin' gun not and not a wild guess - well in my experience that's how it turns out even after getting frustrated and throwing parts at it in hopes that I'll guess right.
For others reading, most of these symptoms will also apply to a factory type jeep only you need to increase the scope of your search to include the steering itself and potentially suspension components. Remember that a front diff with a trac-bar (also known as a 5th link) will make the diff itself move while steering causing the other suspension components to shift slightly. Again this is unlikely going to result in a clunk while testing your steering in the driveway but it may cause one under power.
That's all I can think of at the moment. Let us know how it goes!
Matt