Based on the
2015 Ford Towing Guide, the Explorer is only rated for 2,000 pounds WITHOUT the Heavy Duty Trailering Package.
When my sister bought her 2016, we simply made sure it was equipped with it.
However one of my friends had a Ford Flex without it. My research then indicated the only difference on the Flex, was the Heavy Duty Trailering Package added an Engine Oil Cooler. It's likely that your Explorer is missing this component, but Explorer (Sport and Police Intercepter Excluded) also had smaller brake rotors than the Flex, and might also lack a 2nd transmission cooler, but I have not confirmed this.
I know you already have that Explorer, but you'd be far better off selling that one, and buying a Sport or Platinum model with the 3.5L EcoBoost. This will tow 5,000 pounds effortlessly.