Transmission slipping

What is your question?

Was driving down a dirt road about 40 miles an hour when out of nowhere felt like the tranny slipped into neutral but it was still in gear. was able to get it home but I had to be at about 4,000 RPMs for the truck to keep moving about 20 mph does this sound like transmission just went out or maybe something inside of the transmission is this common?

How long has your 1998 Ford F-150 had this problem?

Just happened