vibes vibes viiiiiiiiiibes, another i got vibes question

Hi yall, 91.5 4.0

I recently did the swap from my weird non c-cliped disco to the non-disco and I've got vibes like a u-joint is going out. We put the new shaft in and it immediately vibed down the road. We put in the old shaft because we found out that we needed seals! So, put the old one in and it rides smooth as silk, but now that I've put it back in with the seals, it vibes like it's going to explode! I don't know what it can be, I feel like if it's a u-joint then it should be vibing no matter if it's disco or non-disco. Any help will be great!

A) the ujoint in the new shaft is shot and causing vibes, or (B) ujoints in the front driveshaft are shot and causing vibes now that the lack of a disconnect lets it spin full time...:wave:

Front shaft out of balance? The non disco front shafts werent balanced from the ones I have seen. I have a couple of each in the garage. Discos are balanced and non discos are not balanced.