Perhaps a moderator will move this thread to the Transfer Case section of the forum.
I've had various problems with U-joints. They absolutely do not last forever.
I've gotten to the point where if I can get a suitable joint, I install the ones that don't have a grease zerk. The caps get loaded with quality chassis grease, stuffed into place assuring that the joint flexes easily/properly, and then I leave 'em the hell alone for as long as possible.
They tend to last longer than the joints with zerks--I think that there's dirt around the zerk that gets forced into the joint when they're greased.
The most-recent U-joint work I did was on an aluminum driveshaft. Aluminum driveshafts need "special" U-joints, that is, U-joints with coated caps to prevent corrosion between the steel cap and the aluminum yoke. I could not find a coated U-joint that didn't have a grease zerk, so I cleaned the joint, installed my usual chassis grease, installed the joint so the zerk is under compression when moving forward, and now I just pretend that there's no zerk.
The sliding splines you mention have not given me any issues, but I have seen them on other folks' vehicles, very worn-out, to the point that they show excess play--I can push the shaft up and down at the splines, which has got to be REALLY bad for driveshaft balance.
