1998 Ford Windstar Transmission? Electrical?

Several months ago, my Windstar started to do something very strange. It will not let me drive it any further than about 20 miles away. Once in that town, I can drive all over the place, I can shutdown and restart the car, I can go thru drive thrus, I can then drive the 20 miles back home without issues. I cannot drive my van any further than that. I cannot follow someone else (drive the van in someone else's style). No one else can drive the van. I can accelerate hard and do 60 or a little more but I cannot pass another car that is doing at least 40. If any of the above rules are broken, the van will suddenly feel as if I have hit a brick wall and drop from 5th gear to 1st instantly. From that point on, unless you let the car sit for 2 hrs and cool off, it will not go out of 1st gear. We have changed all O2 sensors along with a couple of other things. Everyone is bewildered. Everyone thinks it is Electrical as opposed to mechanical.

There is also hesitation in the lower gears as it changes gears, almost a shudder as it changes up to each gear. All of these issues started in December and we have not been able to resolve. The engine light had been on since we got van a year before the engine light had been on but after changing the 3rd sensor, it has not been on since.

UPDATE: We replaced battery Thursday Night because it died in a parking lot. Today, we know that the alternator is bad. I noticed on my drive today that the battery light was flickering on and off until I reached about 50, then it would stay off. Unfortunately, it was completely dead once I got to where I was going so hubby will be replacing the alternator on the street tomorrow. He is now thinking that the previous problems were a cause of the above issues. Any thoughts in any direction?

Any ideas?