Replaced all 4 u-joints on the 80 this weekend. Not a terribly complicated project -- sledge hammer and some old sockets are really all you need. Only one of the u-joints felt like it really needed replacing, but I replaced them all anyway, since I had purchased them. The bad u-joint felt a little notchy when flexed back and forth. Chances are that I could have pumped in some grease and it would've been fine, but as I said, I had already purchased the parts...
Gotta love Toyota parts -- the cups were pre-filled with grease, which held the needle bearings in place. It's been over 20 years since I replaced a U-joint -- I think the last time I did this was on my sister's '70 Nova. I don't recall the cups being pre-filled with grease on THAT!
It will be interesting to see if I have solved my mystery clunk. So far I've rebuilt the front axle and replaced all 4 u-joints. This is the "shotgun method" of fixing problems -- just keep replacing parts until you solve the problem.
Still need to re-align the front end and replace the front brake pads before I hit the road to the GGG.