Well, got the driveshaft in after all new u-joints and a new carrier bearing. Took it out for a test drive. It was alright up to 30, where it developed a bit of a vibration. At that point thought "Alright, we'll see." By 40 MPH it was very noticeably shaking interior and exterior mirrors and by 50 it was borderline filling rattling and getting worse so I never hit 60 MPH.
Took it off, doubling checking phasing, that I clocked all the yokes with my witness marks, had the zerks in the right place, checked each bearing cup for clearance (Spicer included 3 thickness clips to shim them). I put it back on, checking everything I could think of. I even followed the left-right adjustment of the carrier exactly like the FSM shows. It was way more perfect than I've ever tried to make a driveshaft installation. But it was clearly out of balance.
Threw in the towel and took it to the shop. Driveline Service here in GJ, the place Hugh at Safari Unlimited used to use actually. He put it on the balancer and it would not even make one full revolution without flopping around. The slip yoke splines were toast. Stuffed them full of grease. They are not designed to be greased so that was throwing darts and if it had worked (it didn't but damped the movement on the machine at least) it would have lasted about 5 minutes.
So
that got done. They use Neapco parts instead of Spicer. I don't know that it really matters, it does look nice and is greaseable, too. Took a little less weight to balance. From the factory it had a couple of sizable hunks of weight that it doesn't now.
It's much better, not Cadillac smooth but quiet and reasonably smooth and doesn't feel like it's trying to pole vault the driveshaft. I think having the slip yoke at the top by the carrier instead of at the axle like Toyota used to do might not be ideal. It seems to me the splines are the most likely to develop wear and it will be amplified by the rubber-isolated carrier rather than the third member holding it. Not sure all the shaft variables a driveline engineer might consider but maybe it has to be done this way when they eliminated the double cardan (which IMO is just being cheap).