I don't play the numbers game, but I can relate my personal experience.
I towed an old 19 foot travel trailer with a 1976 Ford E150 Club Wagon, 351CI V8 engine, auto tranny. We went into the mountains of Central Montana on 2-track dirt roads, we went over the Continental Divide several times on paved highways, with no problems. No WD hitch, no "sway control", it just worked.
Prior to that, we had a 1975 Ford E150 Club Wagon, and towed a 15 foot camp trailer in the mountains of North Central Wyoming. We even towed it to Tucson and back. Again, no WD hitch, no "sway control", no problems.
OP, if your van has P rated tires, I would replace them with LT load range D or E tires, and go for it. If you (or the van) are not happy with the towing performance, then you can think about re-gearing the differential to 3.73 or 4.10, or replacing the van.
Good luck.