Thats a filthy greasy thing there at the U joint, hard to imagine its held up by rust, so it would have to be a mechanical sort of resistance. Someone previously mentioned the end being peened from rubbing on the U joint bolt...can't be as the ends of every drive shaft I have seen have a substantial taper to them,on the end, like a 1/8" at least. Do you have the speedo turtle off ? maybe have a look in there. This is a puzzler !