When I stop and take off again, my brakes sound like they are still partially engaged and then release as I pickup speed. I have bled them and replaced the driverside caliper

I would jack up each wheel and see how they turn. The back wheels, with the tranny in neutral, should not drag but you will be turning a bunch of gears. If you have a locking rear diff jack up both rear wheels. The front wheels will drag a bit on the brake pads because disk brakes do not retract, but the wheel should be fairly easy to turn. Hopefully you will find the offending wheel. I'm curious as to why you replaced the caliper?

I thought that the caliper was dragging.

I had an accident with my 2019 Ranger. It seems when I applied my brakes the anti-lock released my brakes and I ended up hitting the person in front of me when he nose-dived his vehicle. I hit him at about 10mph or less. and the damage came up to over $12,000.00 (pretty chinsy front end). If my brakes would not have released I would not have hit the vehicle, but because it DID release, it cost me an increase of my insurance, even though when the police officer reviewed my dash cam stated that it was not my fault. Unfortunately in Arizona the cops are a bit shy on issuing tickets because we are considered a "No-Fault" state. I was wondering if anyone has heard of a possible brake issue other than mine on something similar?

The other day when I had to step on the brakes hard, the brakes locked up, when I let off, everything was fine. I wonder if my ABS is bad

Well ABS stands for "ANTI-LOCK Brakes" which means the brakes are suppose to keep from the brakes locking in cases of hard stops, which also means that you are at a greater risk of having an accident, unfortunately in my case they did anti lock but they also released my brakes totally which was why I had that accident. I was going less than 10mph when I hit the vehicle in front of me (when he nose dived his vehicle). At that time I wished my brakes would have locked up and I would have not even come close to hitting that vehicle. To me I have always loved the Ford Rangers (since I have had 4 of the 1990's Rangers), but after this incident I'm not sure I want to keep this Ranger, it has been nothing but bad luck. The next truck I buy might be the Freightliner Pickup truck that just came on the market. At a starting price of $8000.00 bare boned and with the options I want it goes up to about $15,000.00 or so, it sure beats the $48,750.00 price tag on my Ranger.