Drivetrain sounds like a playing card in a spinning bicycle spoke

[TJ Highlander]

I was out off-road today on a trail that's barely a 2 if even that. I never even took it out of 2 HI. On the way home, I heard a sound that sounds like it's shedding rocks from the tire treads up against the bottom of the tub. Nothing unusual there. Except that I hear one dull "pop" that I also feel through my shifter. Then when I let off of the gas (whether in gear or out of gear) I hear a sound coming from under my TJ that sounds like a playing card in a bicycle spoke (for those that did that trick when you were kids). When I apply any amount of gas at all, it disappears. The sound is accompanied by feedback through my shifter and it varies with my speed.

When I crawled under it I see nothing unusual or out of place.

Nothing seems different about the way it's handling or running in any way.

My first thought was the u-joints, but they appear to be solid and the caps are all in place.

Other ideas or suggestions?

Thx!