Between 52 and 65 or so. These are the things I've changed or checked

run out on axles .004
brake drums replaced
wheels tires switched with known
rear axle bearings replaced
shocks replaced
u joints replaced
balanced driveshaft

trans angle -1.8
driveline +1.6
pinion +2.0

I read it needs a minimum 1.0. Is this my vibration?

Had one at 70 mph on my b-body. Too much pinion angle. Reduced from 6* to 4*. Much improved. Prior to 6* it was less than 2* and it vibrated. Had to find the sweet spot.

Try putting the car on 4 jackstands, under the suspension, and run it up to see which half of the car the vibration is coming from If it is in the drivetrain you may be able to see it Please let us know what you find

What Kevin said & I'd also check the amount of front yoke end play into the trans (want 3/4-7/8). I would then shim the pinion up or down with angled perches & see if it changes

The slip yoke bushing feels fine, no leaks etc. There is about 1" of end play. I didn't say prior that I have added shims under the trans mount, moved the vibration up in speed and more significant. Also shimmed the diff down 4 degrees and made it worse as well. I'll try the jack it up and run it w/o the tires and see if I can see where it coming from. I did follow the truck down the freeway and could see the rear end bouncing when the vibration was present.

Well I give up, shimmed the rear up and down, both ways makes the vibration worse. Shimed the trans up, got worse. Balanced all 4 tires/wheels just in case. I can make it worse but can't make it go away. Odd thing is you can barely feel it when it's on jack stands. Tire and wheels made no difference. The vibration when on jack stands is barely noticeable no matter the changes made. I wondering if it's in the diff? It feels fine when your turn it? I did have the cover off when replacing the bearings and everything looked good.

I was wondering about the pig myself especially since you can see the housing vibrating. Otherwise I'm still thinking shaft balance/ujoints/ext housing bushing.

Once upon a time in my youth a street and strip Olds powered B modified production 55 Chevy with a B&M Hdrostick.

Had tendency yo blow rear U joints and a what felt like a drive line generated vibration at 70 to 80 mph

One day jacked rear of car and placed our standard (yes I know) cement foundation bricks under the chassis, crawled under car and had my friend start the car and put it in gear.. Living on the edge,,,we are all lucky to be alive.

The rear U joint looked as a Mixmaster the way it was going up and down. Turned out that I had likely at one time tweaked the pinion flange in one of my U joint failures and it was running the rear of the drive shaft in an ellipse and not a circle. Replaced flange and was well.

I had a Dodge van that vibrated around that speed and it turned out to be a dented driveshaft. I replaced the driveshaft and all was good.

I read on here years ago about proper belt tension can help.

You said you changed tires and rims. but that could be the problem . Balancing the tires don't fix "out of roundness" , You just might have put on something that had the same problem but not have been noticeable on another vehicle. If the diff is vibrating , most likely its the tires or the shaft. Do a few things first that don't cost too much time and money .
Put a hose clamp on the shaft near the pinion , mark where the screw is , rotate about an inch at a time until you find the sweet spot. If you find a position where the vibe is less but still isn't gone , add another clamp. If this fixes the problem, have the shaft replaced with a known balanced one or have your original balanced.
If this don't fix the problem, go back to tires and have them checked for eccentricity or out of roundness at some qualified shop, not at Wally World or some other fast part outlet . Let us know how it works out .

yoke on the rear end.