Originally Posted by yogarda

Alrighty then after many, many rainy days in which I could not work, we have the solution. I learned something very interesting from a local 300D lover that I met by chance in a Tractor Supply Co parking lot while buying a gallon of PB blaster to implement my grand idea. He gave me his card and told me to call him if I ever needed parts cuz he has a fleet of W123s.

I decided to soak my u-joint in PB blaster overnight to try to get the thing to free up and move - if that worked (which it did) then soak it in 80w gear oil for a couple nights so the places where dirty grease had been could be "revitalized" with some decent lubricant. All by capillary action. A shot in the dark. I figured if I could get the thing moving nicely, maybe it would drive okay for a month and then I could get a rebuilt ($365) with confidence.

So I did that. After the three nights, the joint in one direction felt like new, but the joint in the other direction still had a bind in the middle of its range of motion.

Now all this time I thought the binding was a pit worn in the joint - not in the caps but in the part the caps cover. However, I called over to the gentleman I met in the parking lot to see if he'd give me an opinion on the now-much-better movement coming from the joint. We were able to hook up and he taught me that the binding was not wear, but that the needle bearings in the caps were rusty in some spots.

Now here comes his brilliance: He had me hold the joint flat on his vice while he grabbed a deep socket that fit nicely into the where the caps are seated. Then he starts whacking it with a hammer. You could see the cap he was hitting sink down into the u-harness. Then he'd have me turn it over and whack it back towards the other side.
He did this about 5 times on each side, about 3 hits each. Tested the movement for smoothness after each flip. You could see wet rust oozing out of the cap seals. After the 5 times, this part of the joint also moved smoothly, almost like new.

Install the shaft, drive away... no more shaking, no more thumping, gun it as hard as you like.

So the answer to this mystery was a dirty u-joint. Never knew you could practically restore one with a hammer and a dream.
I imagine the joint itself may have more longevity if I had never soaked it in PB Blaster or gear oil. Maybe if the original grease were in there during the whacking it would take longer for the rust to ooze out but it would last longer.

Anyway, I'm gonna get as much time out of it as I can and I'll close up this thread when I know when it needs to be replaced with a rebuilt.

THANK YOU EVERYONE for your advice and support.
-Garrison