As pointed out there are many potential causes or factors involved with sway. So you are going to have to do some learning and figuring.
For learning you first should understand trailer stability. Your trailer will have a Center of Gravity (CG)which is the point where the trailer balances front to back and side to side. The location of the CG is very important to the stability of the trailer. If the CG is behind the axle the CG will try and get in front of the axle and you will get sway (or the "ground loop" you experienced). Thus you want the CG to be in front of the axle, and the further in front the more stable the trailer will be. This is the basis of the 10% rule for putting trailer weight onto the hitch. Understand that 10% is a minimum and more weight on the hitch corresponds to a CG further ahead of the axle which will be more stable.
Watch your weight and loading carefully here to keep that forward bias. Sounds like cargo needs to go up front, especially the heavy stuff like the cast iron Dutch Ovens
Next learning is that sway is often a harmonic oscillation (airplane guys call it flutter). The problem with harmonics is they can add/amplify energy as you see with a sway that starts small and keeps getting bigger. Everything (your trailer suspension, your truck suspension, etc) has a natural harmonic frequency where it likes to bounce and you operate in it at your own peril. However change a parameter and you change the harmonics. For example speeding up or slowing down changes the harmonics but travelling at just the wrong speed...
Here is a harmonic scenario: the trailer moves right taking the truck with it. The truck moves because there is movement in parts of the suspension: tires, springs, suspension bushings, etc. These parts absorb energy like a spring until there is a balance with the push of the trailer. This stops the movement at the hitch but the trailer still moves. But since the hitch is not moving the trailer begins to be steered by the hitch. But the trailer also has movement in springs and tires and bushings. Eventually they all get loaded and the trailer stops moving to the right and in fact starts moving back to the left. This starts a new cycle to the other side. But going to the other side has some extra help from the wound up spring action in the trailer and truck. So the deviation to the left can be a bit stronger than the deviation to the right was.
Note that if the trailer "spring" unwinds before we get to the right limit we avoid the amplification. Similar with the truck. What we want to avoid is having the same back and forth frequency of everything: the trailer, the movement and the truck.
Stiffer components have a higher natural frequency and softer are lower. Sway tends to be a low frequency harmonic. Thus stiffer springs in truck/trailer will help. Stiffer shocks will help. Stiffer tires on truck/trailer help.
Note that bigger trucks often come with stiffer springs and shocks and tires, thus the common perception that they tow better. The larger truck is effect but not the cause.
Your Ranger, like most vehicles on the road today, was designed for a smooth comfortable ride with a lower load. When you add weight it lowers the natural frequency and makes it wallowy. Wallowy is bad for towing. You will want to stiffen it up for towing much as you would for hauling (BTW: you say it drive great but how does it drive with a few hundred pounds in the back of the bed?). As noted max out tire pressures. Perhaps switch to higher load rating tires. Stiffer springs or helper springs in the rear would be good. Look for stiffer rear shocks too. Make it just a bit harsh without the trailer or load in the back.
It has always puzzled me that trailers do not have shock absorbers... they would certainly help here too.
I used to tow utility trailers with a Ford Mustang which most would consider a very poor tow vehicle. It handled towing great. Of course it was lowered with stiff springs/shocks and had oversized brakes and a torquey 4.6L V8 too. All stuff you would find on a good TV.