Driveline tapping while decelerating

This just started happening this past weekend after a 150 mile round-trip excursion. It seems to happen in both two and four wheel drive and is only audible during deceleration. It sounds like someone is lightly tapping a hammer on the underside of my Jeep. The tapping is louder when braking and seems to get even more pronounced while braking in a turn.

It's an '01XJ with 167k miles, 4.0L, AW4, NP231, 8.25 rear and D30 front. I suspect the u-joints are all original and I feel they may be my culprit. Just wanted to get a second opinion before I crawl underneath it tonight and start wiggling stuff to see what's loose.

I checked for u-joint slop in the rear driveshaft when the vehicle still had all four wheels on the ground and was in park. I couldn't get any slop out of the u-joints and there was zero slop in the rotation of the rear driveshaft. I checked the front axle u-joints too and I couldn't get those to budge and they seem pretty solid. There appears to be a little bit of rotation slop in my front driveshaft axle, but no more than my '98XJ with 74K miles on it (the '01 has about 167K and I believe all original u-joints). The transmission mount seems to be a bit loose, but nothing terrible. I've got a brand new one sitting around so I think I'll install it and see if it changes anything.

New transmission mount in but no change. I was able to do some better testing in a nearby parking lot to try and better pinpoint where the sound is coming from. Definitely sounds like it's coming from the front of the vehicle and I can even feel the tapping in my feet. Originally I said the sound will occur in 2wd and 4wd and while this is true, it's definitely a lot less pronounced in 4wd Hi and basically non-existent in 4wd Lo. I do feel like there is excessive slop in the front differential as I can turn it in either direction about 1/2", but the slop seems to be internal and not due to bad u-joints as I can barely get them to budge even when I use a little bit of force from a prybar.

Any ideas or some more tests I can do to try and further pinpoint this issue?