Just to clairify, I am always very easy getting on plane to about mid throttle before It ever gets hammered. I already learned that lesson when I blew the end off the vertical shaft in the lower.
The vibration was felt when pulling back on the stick comming off plane. it is only felt when slowing the boat down slowly. If you flip back the throttle a little quicker while dropping down in the watter I don't feel it.
And yes the pantera's are heavy approx 5600 lbs

Coming of plane slowly will make the prop bite and slip.....That makes the most sense......Coming of fast it probably just drags in the water, i have hade the u joints make weird vibrationes also, but i run a totally different set up

good luck

Coupler?