What's that noise?

So any drive in a vehicle this, ahem, mature can lead to apprehension when a funny noise crops up. I was out on a recent nice day and just enjoying the drive when I noticed a "thunk" sometimes I shifted. Not every time, just occasionally. Everything was working, so I had no idea what it could be.

After she cooled off I slid underneath to look around.  Can't find a thing.  Nothing is loose around the transmission, or the drive train.  So of course I start worrying that my problem is internal to either the transmission, transfer case or rear differential/axle.

First things first, I decided to drain and fill the fluids in the transmission, transfer case and rear differential.  No metal flakes, so all looks nice there, although the transmission fluid level was low.  Very low.  A quick drive and good news is the transmission is very quiet.  I just assumed that continuous hum was normal for a Jeep...

The bad news was the "thunk" was still present on some of my shifting.

A few weeks later I was looking hard at the springs and used the drive shaft to pull myself further under the Jeep...the drive shaft moved laterally about 2 inches!  Not supposed to move laterally at all, just rotate.  This was NOT a good thing.

So I rotated the shaft and tried to recreate that lateral shift; no luck but if I turned it just right I could get a little lateral movement.  That meant to me the u-joint possibly was damaged.

So I removed the bolts and pulled the shaft.  Yep, u-joint had disintegrated.

Top of photo is the original u-joint.  All the needle bearings are gone, and the metal on the one end is so worn it wasn't held on by much.  A quick run to my local parts store and they had the replacement.  An easy install.

After torquing it all down I went for a test drive, and yes the "thunk" was gone.  Also gone was a vibration I hadn't noticed before, but noticed it wasn't there anymore.  She's starting to feel like a reliable and fun driver.

Thanks for looking...