Over the past several weeks I'd noticed an increasing vibration as I drove. It would start at about 30mph and then seemed to fade away as I went faster. However, over time it stopped fading away and became even more noticeable the faster I went. And, it started to happen at slower speeds too. Furthermore, the vibration would go away the second I lifted my foot off the gas to coast, or shifted into neutral. It wasn't at all related to engine speed.
A quick query amongst the JeepForum crew offered the consensus view that this was a classic sign of failing u-joints.
So, on a nice sunny Saturday afternoon in March (there are some benefits of living in California) I crawled under the Jeep and dropped the rear drive shaft. This is easily done with a 1/2" wrench (at least on mine). Takes about five minutes.
Before reinstalling the shaft it is a good idea to clean the mating surfaces on the differential and transfer case yokes as well. Any crud in there may prevent the u-joint from firmly seating—and if it isn't firmly seated on one side you'll have more vibration when you drive than you ever experienced with a failing u-joint! Ask me how I know.
Torque things down and off you go! It's also a good idea to check those nuts a week or so later to make sure they've stayed put.
Final bit of advice: when you pull the drive shaft, be sure to either 1) not separate the two parts (i.e., slide the shorter end off the splined longer end); or 2) before you *do* separate them, mark them using paint, an awl, punch, something so that they go back together exactly as they were. You don't want them aligned with each other in any other way since the shaft was balanced as it currently is.