Need some help with my 1969 C-30

[tincan1966]

here's the rundown of my problem. I have a horrible vibration that feels like the rear end in jumping straight up and down at 50-55 mph and 24-2600 rpm.
History- bought truck 2 yrs ago- had been ditched on LF corner-bent lower control arm no frame damage-have had to frame shop- all inspected and measured righton.
Here's what already been changed(some were updates, some to help problem)
4 new front control arms, upper and lowers with all new bushings shaft kits, balljoints.
new front spindles, rotors, hubs, bearings, etc(converted to disc)
new steering box(old one leaked badly)
new steering parts, tie rods, etc(old ones worn)
6 new tires-good quality 10 ply
6 new steel dually wheels- all tires balance with 3 or less ozs of weight and no excess runout
Changed out eaton HO72 axle for Dana 70(changed from 4.56 gears to 4.11)
All new u-joints, and carrier-had driveline balance checked-good
recently found harmonic balancer broken, but the 427 is a nuetral engine, so no major effect there.
Installed all new mounts replaced flexplate with engine out, also replaced trans output bushing and seal(for leakage reasons.

The biggest change in the vibration was when changed gearing-it happens at same engine rpm, but now at higher speeds. can drive thru it, gets alot better nearer 65mph, but never goes away.

has anyone had anything similiar to this? I am at wits end and have already replaced or updated about anythig that spins! LOL.

PM me or reply here with suggestion, I need to be able to drive this truck.

Thanks in advance!