A quick added note - The trailer shown in my signature weights 7000lbs when loaded, and I pull it behind a 1500 with all numbers well inside the list weight ratings. It blows my mine that your particular scenario is even possible with a 3500!
If you had not posted your weights and calculations I would never have ever guessed it would be an issue, and without doing a single calculation I would have suggested a 3500 was over-kill for that!
As is the case just about every day, I have learned something new by being here on the Z. I 'will' be so much more careful in my future recommendations. This problem although almost inconceivable, is real and goes to show the importance of checking everything and taking nothing for granted.