T800 rear end vibration

Hello all. I have a 2003 T800 daycab that is giving me fits. A little history, truck specs, and what i have tried-bought truck used in 2016 with 600k miles, has 740k now. It has an N14 Cummins, FRO16210C trans, 3.90 rears on AG400 8 bag suspension.

It had super single rear wheels, we quickly changed to duals for our work. Transmission was replaced around 3 years ago. Engine inframed 2 years ago. All new u-joints and complete driveline balanced. All new brakes. All new shocks.
All new motor and transmission mounts. New torque rod bushings. New axle bearings. Installed Centramatic wheel balancers last week. Ride height set at 9" from bottom of frame to middle of axle. Nothing I have done to this truck over the years has changed the vibration at all.

Pretty sure its coming from the front rear end, because when i had the interaxle driveshaft out for its balance and new joints i jacked it up in the shop and got it up to speed. Vibration was there, with just the front rear powered.

Now, to try and describe the vibration. Its a low frequency, pulsing vibration. I can pull the gear shifter towards me, taking up the slack, and vividly see when and how much its doing it. Gong down the road at 65 mph, it will throb the shifter 4-6 times in a 3 second timeframe, be still for a second or two, then throb again. Its terrible bobtailing, bad empty, slightly less noticeable loaded. But 95% of the time, its always there. I say that, because one day last summer i drove home from a western KS feedyard empty, 330 miles, 5.5 hrs, and it never did it once. Shifter was steady as a tree, no throb at all. But most of the time, its there. Some days worse than others, but usually its there.

What I think I have eliminated as possible problems- its not tires or rims, due to a complete swap. Not a driveline issue, or u-joints. With the centramatics not helping a bit, is it possible its not a vibration, but something else causing a throb, and causing the rear of the truck to hop up and down?? Bobtailing you can see the right back of the frame hopping up and down.

I have not been into the rear ends to inspect them, to be honest I am young and not sure what I will find when I open them up and don't want to get in over my mechanical head.

I have multiple videos of this problem, and would love to share with someone who is a genius as these problems. It wont let me upload one here, says they're too large. Contact me if you think you can solve it, and I'll figure out a way to share a video.

Would appreciate any and all opinions and advice. Tired of throwing parts at it, and not changing it a bit.

Thanks

 

Ride height is set to spec given to me by my local KW dealer.

Cannot be a tire issue. Didn't change when I switched to duals. And actually just a couple weeks ago it got a new set of drive tires, didn't change anything.

 

Will try that, hadn't thot of that option.

 

We put on a new harmonic balancer at time of the inframe.

I have thot about a bent axle. But repair shops around me say axle shafts never bend, they just break. ?