no power up steep hill, low speed?

I do a lot of fishing in the back country. My trailer weighs 1,800 lbs and another 500lbs of gear in the bed in the bed. My truck works well on the highway, I can climb steep passes without issue at highway speeds. My transmission temperature usually runs while towing between 180 and 195 and when going up steep passes will reach 215-225 but will quickly return to 190.

The situation that occurred is as below.

I am on a steep gravel forest road that has several very steep passes around 30% grade. My truck ran with no issue with moderate passes. However, when I am climbing a steep hill in 4-wheel drive at approximately 10-15 MPH, I lose all power and the truck chugs (almost like a train would), the check engine light will go on. When I step on the accelerator nothing happens. When I stop and place the engine in neutral it will rev up with no issue. When return to Drive the same issue will occur. I even tried downshifting into gear 1 and still no better. Once I was able to turn the truck around and go back downhill, the engine light goes off and it runs perfectly well.

I have been on this road and pass many times in the past with a much heavier trailer approximately 5,000 lbs and had no issues. The transmission was serviced (regular maintenance) fluid changed etc. No issues were reported by the dealership.