So, I stopped driving the Jeep ASAP on 12/23 when the u-joint play I mentioned late last month suddenly became a loud clunk, noticeable when starting from a dead stop or lifting off the throttle at freeway speed.
Looks like Carl and nrs were right on the money; the U-joint was indeed shot...
Not to worry, though: Santa put a pair of new NAPA greaseable u-joints in my stocking, which were successfully installed today. :)