Huh! Index what? The u-joints don't have to be indexed except for putting them in so you can access the grease zerks. I've replaced so many U-joints I couldn't even begin to count them and never had to re-balance the driveshaft.
You either never realized the vibration or just ignored it or got lucky..
I'll go with you ignored or didn't realize that noise, hum, buzz was the joints you changed..
Not all vibrations are physical to feel.
Every part of a shaft is balanced as it sits, so if you do not index the parts of the shaft as it is, it'll have a balance issue.
Just changing joints can throw a shaft out..
I see it a lot, very rarely do people get it right.
