I believe the issue is the fundamental difference between towing a ski boat with the engine sitting directly over the axle(s) and towing a bass boat with a big outboard hanging off the back end. Before making changes, you should try towing a friend's similarly sized bass boat and having them tow your boat. That would at least tell you if it was your truck and expectations vs. your boat/trailer setup.

I would also try to get your tongue weight and compare it to your friend's trailer tongue weight.

Thanks for the ideas. I really don't want to add weight, the tongue is hollow, and I could add weight. But I think redistributing the weight forward is the answer. I have towed my buddies 21' Skeeter and another buddies 21' Bass Cat recently. It's my boat and trailer. The plan is remove winch and reconfigure it more forward. I have been trailering it with 200# of weight on the nose of the boat , does great with the weight.

You probably only need to move the boat a very small amount to get your 200 pounds more tongue weight.

Yeah, I ran into this when I bought my center console however many years ago. Thought about it for about ten minutes & decided I wasn't gonna use the jackplate, so off it went.

Of course, Murphy's law caught up with me two years when I realized the trailer was rusted from the inside out & I had to get a new one built.

Let us know the out come , I have pulled many of boats never had your problem.

Good up to 75mph??? Hmmm.....
Looking at pics though, it looks like the stern is hanging off the back bunk by almost a foot. Then add jackplate. Yup id say moving boat forward by 6" to 8" will probably fix that problem. That'll probably add 250 to tongue weight.

Hey guys, meeting with a trailer builder in Rockwall Monday AM. Hopefully we can get the boat moved forward and fix this problem once and for all. I will let everyone know the results.
I usually tow at 65-70mph, the 75 mph was a test to see if I did have any sway in conjunction with the bucking. No sway, just buck.
Again, really appreciate the suggestions and feedback!

This makes me wonder how fast others feel is safe while towing. I have a Ranger Z20 tandem axle trailer and I have gone 70-75 on the interstate. It "felt" fine but I'm not sure how fast is too fast. Thoughts?

You need speed rated tires to pull a trailer over 60 or so. Check your rating.

Well guys the problem has been fixed! Rockwall Trailer removed the winch stop, cut it in half, extended it a few inches, and reinstalled it. Moved the boat forward on the trailer 8" total. I have owned the boat for 2 months and yesterday was the first time I have pulled it behind my truck and did not have to constantly watch the rear view mirror or deal with pucker factor when hitting bridges or expansion joints in the road ! Now when I get to the lake I'm not all tense due to a terrible trailering boat and trailer! Best $250 I ever spent!
The great part is the trailer bunks are plenty long and didn't need any modifications.

We use KYB mono max shocks . They make a big difference when towing . Also look at your rear springs - most gm trucks have a 3 leaf setup in the rear - there weak from the beginning - see if there arch is gone. We can put you a HD set on with 5 springs - really helps the cause .

Here is the new improved winch stop. Lower and extended more forward.

That'll work. Now quit carrying on and go fishing!

Nice! I'll keep them in mind if I need trailer work on my rig. Right down the road from me.

Here is the winch stop before, forgot to post it...