I bought a new trailer this sping for my new old 23' SeaCraft. It is a tide water roller style. Anyway my new boat has twin 200'S (see attached picture)_ and a bracket on it. That's a lot of weight in the back.

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As a result the trailer has no tongue weight and starts to fish tail pretty bad once I hit 50 miles per hour. I have been advised to slide both axels back around 9 inches. I have never done this before and aim to tackle it one day this week. I was just going to loosen the nuts and take it from there.
Are there any specific tricks to do this? I would greatly appreciate any insights that you can share with me.
Thanks,
Peter