Wayne,
I would be very wary of towing at speed with the hitch in the rear position along with the low pin weight you have. The trailer will be very prone to swaying with that low pin weight. Having the hitch near the rear of the truck will give the pin leverage to twist the truck, hence the tendency to sway - just like a normal travel trailer. Putting the gooseneck over the ball like a TT would make it even worse.
You may have towed that trailer without realizing it was there because you have a very small trailer, a good stout truck, and the hitch point close enough to the rear axle of the truck, but even the smallest of trailers can manhandle a large truck once sway really gets going. I would not chance it - especially with what looks like a very high center of gravity trailer.
Barney