Chasing drive line vibrations 1974 Chevy c10

Still chasing a drive line vibration at about 50 mph. also around 70. all in the seat or rear of truck, steering wheel all good. See the steps i took in diagnosing and figuring it all out.
-I feel my tires are not balanced still after paying someone to do it which sucks. -
Many contributing factors with a 307 engine that vibrates, 400 trans that's worn , body mounts and those shackles rubbing the bed.
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[@davidpurser3179]

I have the same issue with a 48 ford with a 9 inch. I used a sharpie instead of a screw driver so it will mark the drum

[@MikeTheFixer]

Good idea!

[@Hotrodcowboy58]

Bent rim, or axle,