My boat and trailer weigh 5900 lbs
My tires st205 / 75 d 14 load 100
can support just 750lbs each.
I haven't been able to find heavy duty 14" tires which will allow launching with out drowning the truck.
Looks like maybe you are missing a digit? Load index 100 should mean 1764 lbs, not 750.
Or maybe you are seeing 750 kg (1653 lbs) somewhere? Which would be load index 98.
https://www.prioritytire.com/media/wysiwyg/blog/new/pt-load-index-chart-02.jpg
tbodine, 1760 lb for your size tire would be load range C.
Sounds like you need to find tires in load range E.
Hope this helps,
Charles Brennan