2000 explorer XLT / 5R55E transmission problems

In light traffic on the way home the other day, I started to pass a couple of cars. As I accelerated and the transmission down shifted, it almost immediately
started slipping. Backing off the gas it started to catch and then seemed okay. It wasn't. As traffic worsened it immediately began to slip again while accelerating, either from a complete stop or just a slowing down. The next day after it had plenty of time to cool I tested it and I had all but lost "R" and "D". If I manually up shifted it seemed to do okay until "D". I thought I could baby it until the weekend. Next day, made it to work and home. A little slower but no problems. Later, had to go by mom's stopped to gas on the way. The end. Had it towed home. I forgot to mention at the onset I checked fluid level, color, and smell. All were okay so I bought a filter and fluid. Any chance it could still be something this simple. Any and all suggestions appreciated. Thanks.
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