Re: tires
If your back end is moving around and coming off the trailer when you pull out, then your trailer is toooo far into the water. Try loading with the trailer only 1/2 way in the the water and drive your boat on. If you get the front half of your boat on the trailer straight, the rear will settle down straight as you easy out of the water.
Just my $0.02 worth.
Last year I changed out the entire underneath of my trailer. I put in new springs, axles, hubs and 13" tires. I had the little fat 20.5x8x10 tires on it . My reason was because the road ate tires on longer (150 miles+) road trips.
Larry
PS the Facebook picture did not work.