You have a very unstock suspension in your truck.the rear spring hangers are not the stock, the shocks have been moved from outside the frame to the inside. All this to allow for lowering the ride hieght. Note the C notches in the frame.also the springs are not stock width.nothing wrong with all that, worth knowing that as you work on it down yhe road. And yes your problem is with the perches not being secure. Note the rust around the u bolts and mounts. Cut it all off weld on new perches and don't reuse the U bolts, always go new.also make sure the pinion angle gets set right. If it is not , you get a vibration from the ujoint.
