If you are seeing a pretty large increase in pressure, it’s possible that’s being cause by the tire overheating by being at low pressure.
Not a good thing to run really low rear pressure in your DRW. First, you would be way over capacity if you ever had a blow out which could cause the other tire to blow. Two, you are increasing the chance of the two tires making contact with/without a minimal object. Third, you are reducing the stability by using the tire below it’s recommended range on your vehicle.
Frankly, I think the better ride is likely mostly in your head. I do wonder why you bother with a DRW if you are going to reduce it’s load capacity to a SRW and remove your redundancy. Doesn’t seem to make a lot of sense. I’d look into better shocks and/or reducing the air in your airbags.
My F450 sticker says 80 Front and 75 Rear. That’s what I run since I don’t hardly ever drive it empty and 19.5s are different style of tire bead. Conventional E rated DRW recommended pressure on the Rear is 65 though, right?