Vibration When Reducing Speed

I put a brand new Acme 525 on my 02 Sanger DLX today.
The hole shot is improved and it seemed to run just as smooth as the stainless 13 x 13.75 four-blade that I took off. That's good.
The problem is that there is now a new vibration when slowing down quickly like when a skier/boarder drops and you pull back the throttle to turn around and pick them up (Assuming no power turns). There is both a vibration and change in sound under the engine compartment. The vibration almost feels like extreme cavitation. It only seems to occur when there is back pressure on the prop. If I ease back on the throttle, keeping forward pressure on the prop, but still slowing down, it doesn't happen.
I removed the old prop with a prop puller. I had to tap is about 40 times with a regular hammer to get it to pop loose. There was no visible damage to the shaft or keyway.
It was fine with the old prop.
Anybody ever experience this?

Yeeeep! I'm running a 231 on my 98DLX and it did the same thing ... I'm not really sure why it does either.... I went from the stock prop to the 231 .... Stock prop I had no vibration on quick decel, now with the 231 I wouldn't call it a vibration but you can definitely tell a difference in noise (maybe a little tiny vibration).... Would be nice to know why it does this!

And it's not the shaft because I have a brand new one on my boat...