It actually sounds like the Plug inside the Joint is leaking or displaced.
If you strip down the Pot Joint, you'll find there's a Plug, much like a Freeze Plug, inside it to blank off where the Shaft Hole is. These can leak.
I have had great success in cleaning all the parts (super clean) and re-assembling with Loctite 243 on the Plug.
Well I popped those out, cleaned both surfaces with acetone, and put them back in with Loctite 638. I had some lying around from another project but it sounds from their website like it might be an appropriate product for this application. I've also replaced both diff seals so fingers crossed! Will try running it before putting the hubs and everything back on.
