Looking at both vids it looks like it is topping out the shock travel making the rear porpoise up ad down
Did you compare the extended shock lengths on both sets of shocks? If not, if you can , do that now. If your new shocks are shorter at max extension than the old ones you may need to use shock extensions to help fix the extended length. If this is the case I might have a set of them, let me know if you need them or not
Try removing the lower shock retainers and slip them off the lower shock mount and see how much more down travel you have on them, then try jacking the back of the car up with the rear end hanging down with jack stands under the rear sub frames and see how much rear shock travel downwards you have that way also. Please let us know what you find on this
What does your rear spring shackle angel look like with the rear end hanging down all the way?
It ET better in the 60 before so you know it will do it again with the proper fixes