Well I don't know about the alignment either. I did notice in my sons nissan manual that they recomend rotating the propeller shaft 180 degrees if it vibrates and is out of balance. If that does not fix it they say to replace it. Personally I think I would try rotating it 180, test then 90 more test then 180 more test then give up or use the position that vibrated least. So basically test in all 4 positions.
On my 57 I had a shop replace my transmition once and I watched them drop the bearings out of the rear U joint. Seems like they were needle bearings. They picked them back up and re assembled them into the bearing and it is still working 15 years later. But then it has only gone about 5 thousand miles in that time.
Sounds like it only vibrates in 4th so at high revs where you might expect it. I would vote for doing a 180 degree rotation o it since you do not know where it was when you took it off. You might want to mark where you have it now so you can tell where you are moving it to for testing.
But like I said I do not know much about the alignment thing for the driveshaft.
