Not so much the biggest load, as maybe the most under-powered Ranger tow. In '86, I dragged a ~20ft runabout boat from the So Jersey coast to San Jose, CA. Heavy boat and trailer. Six cylinder Volvo I/O drive. The truck was an '83 Long bed Ranger with a 4 cyl. I assumed it was a 2.3L. Oh, no, the dang thing was a 2.0L! VERY fancy truck. Plush, even. Geared for NJ, apparently. They don't have hills there.
I had my choice of two Rangers, but Dad thought the fancy one would sell quicker in CA. Au contrere. I went back for the '83 N/A diesel Ranger after arriving home with the boat. It ended up selling first. Everyone wanted that diesel truck. I should have turned around in PA and swapped trucks.
When I stopped for gas, I had to make sure I was not headed uphill, because it wouldn't move. Could NOT stop on a hill, or I was in trouble. Had to tiptoe across the country on I-80. Springtime, but hit snow in WY. Mostly downhill, which added it's own thrills. It took a while, but it could eventually get that boat up to 65mph on level ground.
When I got that diesel to CA, I hooked up that boat and dragged it around the neighborhood for comparison. It would flat yank that thing from a stop. Very impressive. I was pissed that I chose the wrong truck to drag that boat with.
Very stressful trip. Only worse one was towing my '57 Chevy convertible with the big block, from near Philly down to the Jersey coast. Towed it with my '65 Mustang Fastback 302/4spd. A couple times the front end chatter of the Chevy would cause both cars to skitter off the road in corners. I should have been towing the Mustang, but I think it didn't have enough bumper to hook the towbar to. That was in '71, so details are a bit fuzzy. Hmm, the fact the Chevy didn't have a trailer hitch might have been a factor. Maybe a 1k lb weight difference. And at least 400+ hp difference.