Bill, if you have a DRY WEIGHT of 11,500, rest assured that you are pulling a lot more than that. I would agree with those that say your hitch/kingpin need adjusting to put more weight on the truck. HOWEVER, the tires only inflated to 55 lbs worries me for that much weight. I think you need to double check the tires to ensure that they are correct for your weight and also double check the inflation. Doesn't seem right.
The PROPER way to determine pin weight is what most suggested: Truck w/o trailer weight, then weight the truck ONLY with the 5er hitched and loaded like you will travel with. DO NOT put the whole rig on the scales, only the two truck axles.
Having towed a 5er with a dry weight of 9950 with a 250 diesel (total weight of the trailer was 12,450) and with the proper tires, I found that if I venture up to 70, I began to sway. I adjusted the hitch and it went away.
IMHO, that 3/4 ton is too small for an 11500 dry 5er.