Well, on the way home yesterday from work, I noticed a significant vibration between 60 and 80 KPH. Was enough to rattle my fillings, so I decided to get on it right away.
Pulled the rear propeller shaft, and sure enough, the rear most u-joint was seized. Went on a hunt for a hardened 17mm socket and my BFH. Used a pair of needle nose pliers to get the clips out, 2 of them broke due to rust, but the new u-joints, which I've had sitting on the shelf for several years (just in case, you know?) had all new hardware with them. They also had a grease zerk included which threaded into one of the caps, instead of into the main body of the u-joint, which threw me for a bit of a loop. But got it installed. (keep in mind if you have this, there is a special clip for that cap, the normal clip would interfere with the grease zerk)
Went to town on my bench vise, and disassembled the yoke from the shaft. There is a bit of 'finagling' required to get it to install properly, and you have to be mindful not to let any of the needle bearings inside the caps move around or fall out. But after about an hour of wailing on the yoke and the drive shaft, I had a brand new u-joint installed.
No more vibration.
Didn't get much life out of this u-joint, but its mostly my fault as it is a serviceable u-joint with grease zerk and I only remember lubing it a couple of times. Need to fix that.