[HO455]

As far as I know leaf spring trucks did not come with the hanging inline style. (See photo below I have stolen from a post by chev-obsession, thanks ) You should have one of the horseshoe/sandwich pillow block style carriers. I know that there were at least two versions of that style used. Which style seems to depend on what the GVW of the truck was rated for. That being said I have never compared a C30 carrier with a C10 or 20 carrier, so there may be even more versions out there.

There are a lot of contributing factors to vibration. I would start with wheels and tires. Then if I owned the truck and I knew the drive line had been gone through (Including the sliding joint in the rear shaft) my next step would be the rear end if nothing was known about it. (This also assumes this is your average 75k mile plus truck that wasn't stored for 50 years in a heated garage.) I cured a good portion of the vibrations on my Burban by going through the rear end.

You didnt mention which transmission you have. The output bearing of the transmission can contribute to vibrations too, more so on manual transmissions.

I hope this helps and good luck.